No Mercy's pretty much the same as far as I've seen, except for the main building of the finale. The ground floor is empty except for the radio, the back staircase is now outdoors, the supply stash is on the the roof, and the second story "shed" is entirely gone. It's a whole lot more exposed and there are no closets anymore; IMO it's what it should have been all along.
The L4D2 equipment is good to have without being game-breaking. It feels like it's always been there. And there are fireproof zombies and construction worker zombies in appropriate areas.
Looking back, the biggest gameplay difference between the two games may very well be the Spitter. All the parts where I've suddenly, viscerally realized "this ain't L4D1 anymore" were moments where we were trying to hold a corridor or other such chokepoint and suddenly found ourselves knee deep in goo.