Gaining a level (see page 27 of the Player’s Handbook) is the most significant reward the game has to offer, but even that reward has its own tidal rhythm. Characters gain new attack powers at odd-numbered levels, and they gain new feats, ability score increases, and global adjustments to all their attacks and defenses at even-numbered levels.
So this gives you a total of +15 additional points to ability scores by the time you reach level 30. Which means if your fighter could have a 33 strength! That's freaking incredible, how strong would that be in DnD terms, like lifting elephants or something? Or your elf ranger could have a 35 dexterity, your a freaking mongoose now! :OMG!