I don't have any idea what the planned level cap is, or if there will be a prestige system, but I've got maybe ten hours into the beta, have a level 20 Ares and a level 10 Zeus. I'm a very average player, at best, so I'm sure there are others who are leveling up much more quickly. The most egregious violator of leveling too quickly was Light Box Interactive with Starhawk, where the best players made the level cap at 50 within a week of the release, and while GOW:A MP isn't NEARLY that bad, I think we need to work for it a little harder.
Borderlands 2 is perfect in this regard, imho. Game's been out since September 18, and I haven't hit the level cap of 50 with any character yet (I have about 70 levels over 3 characters at this point). You REALLY appreciate those later levels when you get them, and what you get with them.
My two cents. Thoughts?
If the cap is 100 then the current system would be fine, I think. If it's 50 then I agree it's much too fast. And I totally agree that the higher levels should be profoundly difficult to attain.