Players not asking for XP, or being interested in plot development has little to nothing to do with what system you're running a campaign in. It has almost everything to do with having good players and having a good DM. If your players had that reaction to your last session, I have to congratulate you for being a really cool DM who hooked them, but don't water down your success by feeling it's due to a system change.
I've been hooked by a DM who ran various games in Alternity, Shadowrun 3rd, and D&D 3.0. Her ability to craft a wicked story and draw players in was independant of those systems, some of which are mechanical nightmares at times.
I'd like to think I've done a pretty good job with 3e, and I don't see anything improving or going downhill if I switched to 2e, 4e, or a deck of cards and a bent penny. Regardless of running 3e or 4e or Pathfinder or GURPS, I'll gloss around 90% of the system, and there won't be a single NPC with more stats behind the screen than story notes.