Progression. The 1/2 level thing is just great. Every character has a sort of baseline which they can then augment, and it just a tangible benefit to everyone as they level up, and makes it much easier for a DM to guesstimate what the party is able to do.
Fluidity. I did a quick delve for 4e and it quite honestly, everything just meshes into a really solid, flowing system. At no point did anyone say "I don't think it works that way" in regards to anything. Although that could be because we just learned the system...
Mobility. On paper it doesn't sound like much, but everyone darting around the map can make for a much more intensive and cinematic game. There's no more standing in 1 spot round after round rolling wads of dice until something dies.
Concise. The rules are straight forward and everything important can be summed up in just a few words. And to avoid irony, I'll stop there.
Viable options. You have to actually try to no be effective in 4e. From what I've seen no matter your level, class, race, opponent, feats, powers, etc. you can still contribute in meaningful ways to the party.
Balanced Options. Tying in with the above, from what I've seen, there really isn't a single class that is so more dedicated to the powers of win and hax than the others. And it seems like that there isn't an over powered combo of abilities, though I will admit that there may be a few in the future with additional releases, it's just the nature of the beast.