Are any 'Whedon alumni' from his TV shows in this movie?
Yes, Jeremy Renner who plays Hawkeye was the vampire Penn in the 'Angel' story 'Somnambulist'. Alexis Denisof who plays The Other was Wesley Wyndam-Price in 'Buffy' and 'Angel' and Senator Perrin in 'Dollhouse'. Enver Gjokaj who played Victor in 'Dollhouse' is the young NYPD cop talking to his superior officer during the final battle. Ashley Johnson, who was also in 'Dollhouse' as Wendy and was briefly imprinted with Caroline in the episode 'Omega' (and played a role in the unaired pilot), played Beth, the waitress who was featured in the final battle.
The main villain is Loki (Tom Hiddleston). This follows the original comics where Iron Man, Thor, Hulk, Ant-Man, and Wasp form the Avengers to defeat him. Loki allies with the Chitauri, the Ultimate version of the Skrull. The Chitauri are the reason the team, renamed the Ultimates, formed in the Ultimate Marvel comic line. The Skrulls are unavailable to Marvel due to the rights being owned by Fox as part of the Fantastic Four characters (including Silver Surfer and Galactus). The "Other" (Alexis Denisof) is either the leader or the voice for the Chitauri. This is an original character, though Herr Kleiser (of the Ultimate universe) would be a close approximation. The giant flying "snake" Chitauri are also original. It has been suggested that they are based on the Midgard Serpent. During the end credits, it is shown that Loki and the Chitauri were in league with Thanos (Damion Poitier). In the comics, he is in love with the embodiment of Death. He is perhaps most famous for using the Infinity Gauntlet (seen in Odin's armory in Thor) to erase half the people in the universe. It is rumored that he will be the villain of Guardians of the Galaxy and The Avengers 2.
The Avengers is a superhero team comic book published by Marvel Comics starting in 1963. It is the most significant general membership team in the Marvel Universe, as opposed to the Fantastic Four, which is primarily a family organization, and the X-Men, which is the superhero team for Prof. Xavier's school for mutants. Those who appear in the movie include Steve Rogers/Captain America (Chris Evans), Thor (Chris Hemsworth), Tony Stark/Iron Man (Robert Downey Jr.), Bruce Banner/The Hulk (Mark Ruffalo), Clint Barton/Hawkeye (Jeremy Renner), Natasha Romanoff/Black Widow (Scarlett Johansson), and Col. Nick Fury (Samuel L. Jackson). Founding members Ant Man and Wasp are not on the cast list, while Captain America, Hawkeye and Black Widow are charter members in the film as opposed to later additions in the original comic series. Also, Nick Fury and S.H.I.E.L.D. had nothing to do with the creation of the original version of the team. However, the team's first enemy in the premiere story in the comic series is Loki, as in the film.