To me that's a good sign in terms of what he could do for this movie. He is capable of making great television with clearly limited budgets and on top of that he seems to have a knack for creating unique, likeable, and most importantly fun characters (Dr. Horrible, anyone?). The ensemble cast he will be handling will have to be all of those things. Can't deny that it is a tall order though--these won't be lesser known TV stars he'll be dealing with (Robert Downey Jr., Samuel L. Jackson, Chris Evans, possibly Edward Norton...some big personalities).
As far as his feature film history goes, I've read rumblings like "Serenity was like a two hour epsiode of a television show." That's a BS criticism. It was based on a television show which didn't attract enough viewers to get another season; how much money do you think he got to make that film? $39 million! That was good shit when you compare it with the budgets for other genre-similar movies (District 9 doesn't count--Neill Blomkamp is on directorial steroids).
Bottom line is I think this is great news.