Also, I'd like to see someone answer this question honestly. (And of course the assumption is that your combination of heroes isn't ineffective comparatively.)
While some characters are obviously better all around than others, if you have a team of 100% the best characters, you wont win against a well rounded team. The character selection and most of the higher level strategy comes in here... A team of carries will fail against a well balanced team, that's just the way things work. There are counters for most character combos, there is no one-size-fits-all character combo, although some combos are easier to play and are incorrectly labeled as such.
In the noob games you played (no offense, it's just what they were) these attributes were hardly exposed. When you're playing against people who are new to the game, you can just stomp everyone with Defiler, you are right. Once you graduate into the higher skilled games, the same strategies that once made you stomp entire teams will make you fail horribly and rage quit. Again, this is similar to TF2... You can pub-stomp all you want, but an organized team (djedi, for example) will fuck your life up.
Also, Ender, shut the fuck up. You're ruining my chances of actually having an intelligent conversation with Virally here... I don't even play HoN that much anymore but I felt that I needed to step in because you're too retarded to talk like a grown up.
Shut.
The.
Fuck.
Up.