We complain because most of us don't have nearly as much time to spend on progression as a lot of other players out there. That we would have to spend time off work/school/sleep to be able to stand up against some of the xp grinders. When a lot of us were performing just fine purely by skill, and now have no chance of catching up.
It's cramping our style.
We don't care that a level 20 has an advantage over a level 2, we can overcome that distance relatively quickly. But when we're sitting on max level waiting for the next big challenge and what happens is everybody else is getting a 1% boost every day that we log on. We fall behind, very quickly. And no amount of dedication is going to catch us up.
It's been 6 months(?) since the release of the Champion System, I have put in about 12 hours of play, because in just the first week I had already fallen so far behind. I literally have no motivation to pick the game back up.
And that's why we don't like it.