Guardians Of The Galaxy is a team with a fairly large roster of characters, many of whom are just kind of "there", while the story focuses on a few at a time and has the others running support. They can't put a hugely expensive "A" list actor into every role, for everyone movie especially if they want to start folding characters into other stories. Drax is a natural to team up with The Avengers to fight Thanos, since that happened with the comics, and the last thing they need is someone who is likely going to start demanding Robert Downy Jr. kind of money especially if they want to use him more than once.
Strictly speaking if they are doing a Guardians Of The Galaxy movie I'm guessing the actual lead is going to be taken by Vance Astro, as he's the human character that ties things together to earth most directly, and also has other Avengers ties via Captain America's shield and so on, a version of him (who can be considered a seperate entity for all intents and purposes) is the Avenger "Justice" for those who are familiar with him, and I'd imagine even if Guardians fails, they will want to spend money casting here because if worse comes to worse that's an actor who has a potential second chance to fill in on The Avengers if one of the existing core actors stops doing them. Early investment and all that.
At any rate, I'm guessing we'll probably wind up with Vance, a boy/girl team playing the two halves of Starhawk (classic version), and someone in a Mocap suit doing Rocket Racoon as the the big focus of the story. Characters like Drax, who really don't do much socially to begin with are probably going to be largely there as supporting cast and to perhaps do some cool fight scenes. People are going to expect fist fighting and stuff in a super hero movie, and Drax despite his abillity to fire energy is one of the more physical members of the crew, and Batista can do choreography well if nothing else which is what the role is going to require. Starhawk in combat is pretty much going to be a total CGI effect, Rocket uses guns and is likely to be another largely CGI effect, and Vance Astro's big power is telekinesis, he pretty much looks at things and moves them (on a pretty massive scale) and the guy most likely to be played by a "serious" actor I'd guess because his fight scenes are again entirely up to the FX crews to make look good, if you can gaze convinvingly like you expect an object to move, that's all the talent you really need for his action scenes for the most part (of course he does do some physical stuff too, but that's the gist of his powers, even when he uses Cap's shield he's supposed to be cheating with mentalism). Batista's job is probably to do something "real" for the token
hand to hand fight scenes.
This is all a guess of course, but an educated one. I'll also say that people seem to be banking on Rocket Racoon being some kind of breakout from this. He's very much a "I can't believe someone made that for a character" character in Marvel who is so dumb he's cool, sort of like say Squirrel Girl, or what they hoped "Howard The Duck" would last as. He's gone through a bit of a revival in awareness in recent years, and I suspect it's related to this project brewing, you'll notice he showed up on the roster for the Marvel Heroes MMO/Action RPG, as well as a character in "Marvel Vs. Capcom 3", so it seems like a bit of planning ahead and seeding the market for his expected success.
At any rate, Guardians Of The Galaxy is cool, and I look forward to it, but honestly if I had to pick "also ran" super teams from Marvel I probably would have liked to see a "Defenders" movie, or even better "Micronauts" though the latter is just to bloody strange and "elseworlds" to have any chance of being made. That and the sheere awesomeness that is Acroyear would probably have too many geek heads exploding, and the clean up bills for the brain matter on the floors, walls, and ceillings after each showing at theaters would probably prevent it from ever turning a profit...
