Firefly's regulars included four actors who'd been guests on Buffy or Angel, three of whom had played recurring roles...
Actually, I think it's the other way around, at least with three of them. Summer Glau of course had a guest episode prior, but I think Nathan Fillion and Gina Torres' recurring roles came after
Firefly - late in Seasons 7 and 4 of
Buffy and
Angel respectively. Adam Baldwin's role on
Angel Season 5 definitely came afterwards.
Oh, that's right. Thanks. I'd forgotten those shows ran concurrently, that Whedon was overseeing three shows at once.
So yeah, that makes my point even better, because that means both
Firefly and
Dollhouse had only one Whedon veteran each in their regular casts, and Summer Glau was barely a Whedon vet at that point, having appeared in only one
Angel episode (although that was the entirety of her onscreen acting experience prior to
Firefly).
Therefore, the assumption that
SHIELD's regular cast will or must include plenty of Whedon veterans is unsupported by prior precedent. There's also the common-sense consideration that lots of Whedon veterans have gone on to have successful careers on other shows and thus wouldn't be available anyway. I'm sure we'll be seeing Whedon vets in guest and recurring roles, but we can't assume we'll see any in regular roles.
I think the show could also be a good platform to then 'spin-off' into other areas. We've heard Cloak & Dagger and Mockingbird be mentioned as possibilities for shows in the past. You could easily see them worked into this show. I could even see ABC do something like you had them appear for a few episodes in SHIELD and then ABC-FAMILY would premier that character's show as a summer series.
Ooh, cool idea -- build an interconnected Marvel Television Universe as an adjunct of the Cinematic Universe.
As a one off series or miniseries then yeah, but I don't see how this can work as an ongoing series. What's the point of having a show set in the Avengers verse when the Avengers are never gonna be in it?
Because it's not just about the Avengers. It's about taking the interconnectedness that's always been part of the Marvel Universe in comics and bringing it to the big screen. The reason the MCU focuses on the Avengers is basically because, by chance, Marvel hadn't licensed the movie rights to any of the main Avengers to other studios the way they had with the Fantastic Four, the X-Men, Spider-Man, Daredevil, etc. If they could get back the rights to those characters (as they already have with the Punisher and may be about to with Daredevil), then we might well see them being added to the so-called "Avenger-verse."
Besides, SHIELD has been played up as a major element of the MCU from the beginning -- to the point that the Avengers were portrayed as Nick Fury's pet project to begin with, unlike in the comics and the animated show where it's the heroes' idea. So one could argue that the MCU is already more the "SHIELD-verse" than the "Avenger-verse." We've already had two Agent Coulson short films and a new short film about two other SHIELD agents cleaning up the post-Chitauri mess, so SHIELD already has an onscreen life beyond the superheroes. And there's an MCU-continuity tie-in comic called
Fury's Big Week that focuses on what Fury, Coulson, Widow, and Hawkeye were doing behind the scenes of the previous MCU movies, showing how the whole movie continuity ties together. SHIELD is the unifying thread of the whole MCU. It's a natural for a TV spinoff.
And of course there's the fundamental consideration that TV shows don't have as much time or money to spend on special effects as movies do. Superpowers are expensive to portray. And TV shows about relatively normal people in relatively normal formats like procedurals or espionage dramas are able to attract a wider, more mainstream audience than TV shows with high-concept sci-fi/fantasy premises and thus potentially get better ratings. It's really not surprising at all that they'd leave the big, way-out fantasy stuff to the movies and focus a TV show on something more "normal" like SHIELD.