Selling points for the half-elf:
1. Racial bonus to con. Good for every character, particularly good for fighters, paladins, and warlocks.
2. Racial bonus to cha. Good for clerics, paladins, rogues, warlocks, and warlords. Note that a half-elf has an advantage over humans for paladins and warlocks.
3. Low-light vision. Better than the normal vision had by dragonborn, halflings, and humans.
4. Three starting languages. Every other race only has two.
5. Only race to get racial bonuses to diplomacy or insight. Very good for social characters.
6. The racial encounter power is custom chosen by the player. Humans racial power is a bonus at-will from their own class, which are not always highly useful(a human cleric focusing on str ends up with a wis-based power), while other race/class combinations may get a limited amount of value from their racial encounter power. A dragonborn wizard, for example.
7. More racial feats to choose from than any other race in the game.
8. The Group Diplomacy bonus is very handy for social skill challenges.
Overall, they've got a lot going for them. With the con bonus and the Dilettante power, they can be almost as good as a human at any class(no +2 in the primary stat or bonus at-will, but more hit points, healing surges, and a complementary racial power), and better than human in some classes. The only race that can compete with them for the title of best warlock is the tiefling, and the only race that can out-paladin them is the dragonborn.