As far as leveling goes, again it is a mix between BF2 and COD4. In COD4 ranks are handed out like candy (I recall a few times where I leveled up 2 or 3 times after one round). In BF2 they take forever to obtain, to the point that only people who have played the game for a combined total of 500 - 600 hours will be near the top ranks (and its likely that most of those won't have the highest rank yet).
Killzone 2 sides more to COD4 in this respect, but it's not as easy. I've played about 2.5 hours in total online in Killzone 2 and I've already leveled up 3 times. I usually level up after about 2 rounds (although that will probably slow down soon as the higher ranks require more points). So I still say you level up quite quickly, but not as quickly as COD4.