I think the OP is overestimating the importance of that second +2 ability score bump, and underestimating the importance of the many other benefits of the human.
Most classes have a single prime ability score. Fighters, for example, rely almost completely on Strength.
The other ability scores may add flavor, but they're not essential to success. Fighters, for example, have a power that pushes a target a number of squares equal to Dex modifier. The difference between an ability score of 14 and an ability score of 16 is 1 square of push. Not a big deal, at all, and there are very few of these types of powers.
Build a few dozen characters and you'll find that for most builds, you care only about the primary score, and the other scores are important mainly for their contribution to defense. So for a Dragonborn fighter, you care very much about the +2 Str bonus, but the +2 Cha bonus you'll come to see as merely "+1 to Will defense".
Now consider the human in that light. They've got the +2 to the primary ability score--any prime ability score. And they've got +1 to all defenses. Given the above revelation of how that secondary +2 is eventually viewed, that's like +2 bonuses to two other scores... plus a bonus to whatever defense is derived from your primary score.
That's huge.
Then you get a bonus feat. You could take Weapon Focus. That gives +1 damage; a good chunk of the benefit from a +2 bump to Strength. If you're a fighter, Weapon Focus is more important than a racial +2 bump to a non-Str ability score. Or consider upgrading from Scale to Plate: that +1 to AC is, again, more important than +2 to some non-Str stat.
That +2 to any stat allows humans to specialize and excel in any class, and the extra feat lets them further specialize. The bonus to all three defenses makes them hard to kill. The extra at-will power and skill gives them breadth & makes them adaptable. Collectively, these traits do a good job of showing why humans tend to be the dominant race in any generic fantasy setting.
Do humans suck, mechanically? No way. Not at all. Are they superior to the other races, mechanically? I think so, but I'm not 100% sure: the per-encounter racial abilities of Dragonborn and Eladrin are pretty hard to beat.
-z
PS: For example, compare a human fighter and a dragonborn fighter. Same base stats of Str 16, Con 14, Dex 13, Int 12, Wis 11, Cha 10.
Both the human and the dragonborn will have an 18 Str. The dragonborn will have a 12 Charisma. For feats, give the human Weapon Focus and Plate Armor. Give the Dragonborn either one (we'll pick plate armor). Equip them both identically: longsword, plate mail, heavy shield. Now compare attacks and defenses (assume 1-handed fighter weapon talent).
Human basic attack : +8, 1d8 + 5
Dragonborn basic attack: +8, 1d8 +4
Human: AC 20, Fort 17, Ref 14, Will 11
Dragonborn: AC 20, Fort 16, Ref 13, Will 11
Over the long haul, that dragonborn will tend to drop before the human, and the human will do more damage. That doesn't make the human better--the Dragonborn's other abilities are cool, especially the minion-clearing breath weapon--but it does make humans at least on par with them.