Well.. substituting Buffy character names for Marvel heroes is not very original..
Anyway.. during his TV years it became apparent that Joss liked some shock moments but used them too much. With Buffy it was original.. Tara's death, the break up of happy couples but when he repeated the same thing with Angel it became somewhat annoying and predictable (though Wesley/Fred was done very well and was really heartbreaking).
I have no doubt that if Firefly had a normal run one of the couples (i guess Simon and Kaylee would have gotten together, maybe even Mal and Inara) would have been torn apart pretty brutally as has happened in Serenity (which was a Free for All since it was the end of Firefly in movie/TV form).
It's one of the very few beefs i have with Joss.. he re-used too many shticks for them be original and unpredictable but i didn't see any of these except for the one shocking death of a beloved character (i guess he can't shake his style completely).
Joss needs to expand his range of writing because it just isn't good writing when the audience just waits for the inevitable death and just knows that the happy couple onscreen won't make it.
Other than that he really is perfect for these kinds of movies.. he strikes the perfect balance between drama, action, humor and suspense and i dare say few would have delivered an equally good Avengers movie.
Despite his flaws i am so looking forward to Avengers 2.