From everything I've seen of 4e so far, I'm excited because it does seem much better suited to sandbox gaming than 3e was. The biggest difference in my mind is that 3e tried to use a unified ruleset for PCs and NPCs, and 4e doesn't try.
Oftentimes in sandbox-style games, I had to do a lot of DM improv because the players opted to go somewhere or do something I didn't have fully fleshed out. In 3e, this meant a lot of times I wound up with "invalid" NPC stat blocks that didn't conform to the RAW. Most of the time this wasn't a problem (how often do you care what a gnoll's bluff modifier is, anyways?). But occasionally players were able to reverse engineer/metagame information about NPCs and realized that things didn't add up. Some players may not care, but I have had some that didn't like that, for various reasons. So for me, the fact that 4e dispenses with all that and just uses separate rules for monsters and NPCs feels very empowering, because I can make stats up on the fly without being concerned about "doing it wrong."
Oftentimes in sandbox-style games, I had to do a lot of DM improv because the players opted to go somewhere or do something I didn't have fully fleshed out. In 3e, this meant a lot of times I wound up with "invalid" NPC stat blocks that didn't conform to the RAW. Most of the time this wasn't a problem (how often do you care what a gnoll's bluff modifier is, anyways?). But occasionally players were able to reverse engineer/metagame information about NPCs and realized that things didn't add up. Some players may not care, but I have had some that didn't like that, for various reasons. So for me, the fact that 4e dispenses with all that and just uses separate rules for monsters and NPCs feels very empowering, because I can make stats up on the fly without being concerned about "doing it wrong."