I agree that the player (strategy) has a lot to do with it, but as was mentioned, some classes are just better suited. I have a level 50 sith marauder -combined with a healing companion there were not a whole lot of non-heroic challenges but did have to skip just about every heroic 4 and some 2s also.... My level 35 bounty hunter was able to solo just about every 2 and a handful of heroic 4s even as far as alderaan and was the only character i have played that was able to solo the heroic 2+ Saving face on dromund kaas (no other quest annoyed me as much as that one! lol)