Also, this is something the 3e+ does better (EVEN THOUGH YOU WON'T WANT TO HEAR IT). Point buy systems. You can get an 18 but it will cost you elsewhere. It also puts everyone on an even keel.
I don't think that is unequivocally a feature. You tend to end up with everyone having nearly identical arrays, just placed in different scores according to class. I think there is something to be said for making the best of what you get, provided you don't end up with an unplayable character. So I like a bit of randomness, both in terms of how high your scores are and which abilities they are assigned to. And I like the idea of choosing class after your abilities have been generated, based on what you get.
But it can be hard to make it work in late edition D&D because abilities play a much larger role in the game. In early edition D&D only the outliers get bonuses, so you have a much larger range of workable scores; for example, IIRC an 8 strength incurs no penalties to your attack, so a thief or cleric can get away with anything from 8-18. In 4e, you want a score of no less than 16 in your primary ability, and some classes really don't do well unless you have an 18. That is not a whole lot of differentiation.
On the other hand, in 4e you can make a workable character that keys off any ability. So you are not stuck with strength for melee characters, or dex for ranged characters, which means you can have a lot of differentiation respecting which are your primary and secondary abilities.
I've played with variations on all of these:
* array randomly chosen from among a list of arrays that are legal under the point buy system
* randomly determining which abilities receive which scores
* randomly determing race
* randomly determining class, from among a short list appropriate to your primary ability score
* use any legal array that includes a 10 and an 11 and assign to abilities; choose class and race; reroll the 10 and 11 score using 3d6 for each (this can get you wildly different scores in abilities that are usually not important to your character, for a little variety)
* 4d6 in order, drop 1 die; if highest ability is less than 16, make it 16; if second highest ability is less than 14, make it 14