Now, one addition to the base races I like a lot is Teiflings. When they were first announced, I have to admit I was leery of making them core. But then, the new race is not that much like the 3.5 Ed one.
To be a tiefling in 3.5, all you needed was to have some fiendish blood in your ancestry. Now, this is kind of cool, but it lacks a coherent racial identity. 4th Ed tweaks this a bit and makes them the descendants of an ancient empire where the rulers made pacts with devils to gain forbidden power. They got it, but all their ancestors were marked by the diabolical energies they unleashed. . The empire may be gone, but the legacy of evil remains.
Now this just seems like a fascinating background to give a race. It gives the DM a lot of jumping off points for adventures. It also gives the player a lot of role-playing hooks without limiting their options too much.
As a disclaimer, being the scion of a cursed legacy is a pretty typical background for my characters, so I may be a bit biased on this.