AThe plain and somewhat minimalistic design of the Enterprise's interior from Star Trek was a financial necessity in those days. As the show was on a very tight budget, the art department couldn't afford too many accessories on the bridge, and production designer Walter M. Jefferies had to keep the design sleek and simple. (The original Star Trek had a budget of $185,000 per episode, equivalent to $1,330,000 in 2013 dollars. Star Trek: Enterprise had a budget of about $5,000,000 per episode.) One indication that this was by necessity and not by design, is that the upgraded Enterprise (the 1701) in the first 3 Star Trek films looked far more futuristic and sophisticated, consistent with the movies' substantially higher budget. Paradoxically, the old, minimalistic design has been hailed by many as an example of efficiency, and elements of the blueprint have even found their way into modern military vehicles (upon seeing the upgraded Enterprise, Jefferies complained that they had "turned it into the lobby of the Hilton").For the Enterprise series, a bigger budget was available, and as the series is situated closer to our own time period (about 150 years into our future), the Enterprise NX-01 was designed to look more like a futuristic equivalent of a 20th century atomic submarine. This means more screens, buttons, lights, accessories and less vivid colors. The emphasis is generally on gadgets and functionality. Also, as NX-01 does not have the benefit of multi-phasic shielding it is covered with polarized hull-plating, giving it a more metallic appearance. Do note that the spaces are generally a lot smaller (the bridge, quarters, doorways, etc. Commander Riker mentioned in the episode "These Are The Voyages..." that the Captain's quarters on the NX-01 are smaller than a brig (prison cell) on the Enterprise D).In short, the 'real' explanation is that our idea of a futuristic ship has changed considerably since the 1960s, now that computer technology has changed our daily life since the 1980s-90s to an extent that could not be predicted in the past. This also explains why a different look was chosen for the Enterprise NX-01, even though it created some visual inconsistency with TOS. Crew members and producers of Enterprise have stated on behind-the-scenes documentaries that they wanted to honour the basic design of TOS, yet they also had to take into account our current state of technology. (In that respect, the 2009 and 2013 movies Star Trek and Star Trek Into Darkness, which chronologically take place between the Enterprise and Original Series eras, evade the issue altogether by completely revamping the look of ships and technology up to the standards that we would currently expect of the 23rd century).As a narrative explanation for this difference, one could say that the 22nd century is unique era in Star Trek lore, since it is pre-Federation; Earth is still an independent world, so the Enterprise NX-01 is technically a terran military ship, and not subject to ship's standards and conventions set later by the Federation/Starfleet. Federation ships are first and formost ships for exploration rather than combat. They can be manned by species others than humans, so perhaps there are regulations as to how tactile controls and screens should look and work. The same goes for overall look in the use of colors. Additionally, the simplified design of the TOS-era Enterprise may also be a sign that controls, screens and functions have been merged together, reducing the need for redundant systems. This doesn't exactly explain the absense of LCD, LED, and plasma HD or 3D monitors in TOS, but perhaps the ship has become more automated to the point that it no longer relies on lots of diagnostic screens to be used by bridge crew (or these functions have been deferred to other areas of the ship, which were never seen). A sleeker, more efficient design could mean an important influence from the Vulcans, prominent members of the Federation who appreciate simplicity. So in ships made after the founding of the Federation, the emphasis is on user-friendly design and efficiency. Advancement is achieved through eliminating buttons instead of adding them and although this appears simplified it is technically more advanced and much more practical.This doesn't particularly explain why more buttons and screens returned in the ships' interiors after the TOS era. It may have simply been due to changes in political climate (increasing external threats, e.g. from the Klingons?) and in policy of Starfleet may have simply warranted a shift from exploration ships to war ships, necessitating altered ship designs. Perhaps the Federation/Starfleet simply endured a shortage crisis during the TOS era, comparable to the 2010's global financial crisis, so simpler ships needed to be build Star Trek always had a way of mirroring events from the real world, and many times even predicted them...Another theory is due to Star Trek: First Contact. Cochran did see the outside of the Enterprise-E when making his flight and Lily did spend time walking around inside the Enterprise-E. Both of these people could have taken those ideas and used them to create the first warp 5 starship.Also, 'Enterprise' could be said to take place in the JJ Abrams universe, which is why the Enterprise in those movies look more advanced. This is implied by a scene in Star Trek Into Darkness, by looking at Admiral Marcus' desk after the attack on the Kelvin Memorial: he has a model of the NX-01 on his desk. The only problem with this explanation is that in the episodes Enterprise: In a Mirror, Darkly: Part 1 (#4.18) and Enterprise: In a Mirror, Darkly: Part 2 (#4.19), the Enterprise crew obtains a starship from the TOS era, which looks like a ship from the Roddenberry universe to the smallest details, and not like a ship from the Abrams universe. It is generally accepted that Enterprise takes place before the schism that caused the timeline to split into two, the Roddenberry universe, and the Abrams universe, and therefore belongs to both universes.This split in the timeline has emphasized another philosophy embraced by some fans, who evade the matter altogether by stating that series creators are allowed to make alterations in (the appearance of) ships, planets and aliens without a mandatory in-universe explanation, as Gene Roddenberry often did when he was actively involved in the series and movies. They reason that designs simply look the way the makers at the time envisioned them, as much as the resources allowed them, and that this may be changed as part of the creative process. These changes are thus considered a product of the time, and narratively non-existent. For example, the appearance of the Klingons changed sharply from Star Trek TOS to Star Trek: The Motion Picture because the budget finally allowed for extensive make-up, facial appliances and costumes. Roddenberry had no intention to ever address this change, although later writers did, and even found a creative solution for it; then again, the appearance of the Romulans has also changed over time (most prominently by adding a cranial ridge), and this alteration has never been explicitly referred to within the series.
AIn the orginal Star Trek series in the 1960s, budget constraints did not allow for prosthetic make-up to be used for the Klingons. However, in the 1979 film Star Trek: The Motion Picture, the Klingons appeared for the first time with ridged foreheads and this design was continued in each subsequent Star Trek film and television series, including the prequel series Enterprise. This made the TOS area the only period in Star Trek lore to feature Klingons without cranial ridges. This continuity discrepancy was finally addressed in the fourth season Enterprise episodes Enterprise: Affliction (#4.15) and Enterprise: Divergence (#4.16).Klingons have by default a ridged forehead. However, in an attempt to create a race of Super-Klingons, some of them were genetically altered with Augment-genes (Augments were super-humans who were genetically engineered themselves, such as Khan Noonien Singh). The genetic enhancements had some very severe side-effects though, and Klingon genes were basically being overriden by enhanced human genes, so not only did the ridges on the forehead disappear, in later stages, the subjects' neural pathways started to degrade, leading to their deaths. One of the Klingon test subjects carried a relatively innocent flu-virus, which incorporated the genetic modifications. The highly contagious new strain became airborne and started to infect other Klingons, who started to undergo the genetic modifications as an effect. The outbreak could potentially wipe out all Klingons, if a cure was not found.Dr. Phlox was abducted by the Klingons to assist them in developing a medicine. He could only create a partial cure which would not kill the virus, prevent infection nor stop the loss of some Klingon characteristics (i.e. ridged foreheads, large stature and relative low intelligence), but it could stop the virus' effects before the symptoms became lethal. This caused several generations of Klingons to have a very different appearance. A definitive cure was finally conceived somewhere toward the end of the 23rd century, when the Klingons took on their normal appearance again.
AThis assumption seems primarily based on a quote from Dr. McCoy in the TOS episode Star Trek: Day of the Dove (#3.7), where he mentions that humans and Klingons have been enemies for 50 years. Since that particular episode was later established to have taken place in 2268, first contact with the Klingons has long been assumed to have occurred 50 years before, around 2218.However, continuity was not the greatest of priorities during the production of Star Trek. Early attempts in the 1980s to create a chronology indeed placed first contact with the Klingons 50 years prior to Star Trek. However, to accomodate later series, the Star Trek timeline has since been re-written several times. It currently has a semi-official status, meaning that many facts are interpretable and open for debate. A good example would be first contact with the Vulcans: although this officially occurred in 2063, a group of Vulcans had already visited Earth in 1957 (Enterprise: Carbon Creek (#2.2)); however, since they never identified themselves as aliens, this never qualified as an official first contact.With the production of Enterprise, the canonical first contact with the Klingons was officially set in 2151 (also the date used in early chronologies). As we can see in the series, initial contact with the Klingons was far from friendly, but restricted to isolated skirmishes and personal feuds, and not a state of conflict between the two worlds. It wasn't until decades later (after Earth had founded the Federation) that hostilities rose to such extents for the Klingons to declare official war on Earth, which must have been around 2218 to conform to Dr. McCoy's statement. The exact events that made Klingons and humans officially enemies have not been stated in any series, but as Star Trek and the subsequent movies showed, it was a de facto state of war with great tensions without large direct combat (modeled after the Cold War in the 20th century).