I'd say that I like
1. The backgrounds. Good amount of fluff, small bonus, comes with four good RP hooks.
2. The archetypes. Fighter, party of four, can give you a decent amount of variety
3. The class flexibility. Lacking in 3.5, but showed up in 4, I like that 5 kept the ability of non-casting classes to do more supernatural seeming things.
4. The naming. I really like that they officially kept to the naming scheme and called it 5, rather than something stupid and non-sequential, like "next". Wait....