I've owned the Nexus One since February, and bought the Nexus S the day it was released at Best Buy.
Front-facing camera ... meh. Not enough FFC's out there yet within my circle of friends/family to make this useful for anything yet, and so far only Tango supports the gingerbread API to even activate it. I was able to video-chat with a coworker on an iPhone 4 to test it, but haven't actually used it for anything.
I miss the trackball on the N1.
Camera on Nexus S is much improved. Comes with a built-in macro mode too which is neat for extreme closeups. The UI is improved too, giving you shutter speed control, focus control, etc., but they removed the zoom feature. Granted, digital zoom sucks, but it was still nice to have.
Gingerbread is nice, but not earth-shattering. ("I look forward to your letters..." - Craig Ferguson) I installed ADWLauncher Ex when it went on sale, but I haven't been completely rocked by 2.3 or 2.3.1 by any means. One thing I do like about 2.3 is that the notification icons for wifi/3G icons turn green when your phone has a connection back to the mothership (google). I also prefer the black notification bar. One thing I HATE-HATE-HATE (and rooted my n1 to get Astro to copy off the root partition for) are the gingerbread ringtones and notifcation sounds. Holy CRAP whose 5-year old kid came up with those?! I copied my N1 ringtones and notification sounds to my NS "usb storage" (what they'd call SD storage on phones with SD storage) and use those instead.
The display is almost too bright, even on Auto or Off modes. Hopefully it doesn't suffer from the massive burn-in my Nexus One ended up with. The extra 0.3" worth of screen is almost undetectable, but the quality of the screen is much nicer than the N1.
Wifi strength seems weaker. I read a rumor that they used a lower-power wifi radio chip in the Nexus S.
And no, it doesn't support T-Mobile's 4G network. But that's not much of an issue for me.
Lack of SD storage was almost a deal breaker for me. Having 1GB for apps instead of 512MB is awfully nice, but knowing that I've only got 13GB for music/photos already feels cramped. I whined about lack of LED notification for charging or when I had an Email/etc, but I got over it within a day or two.
Battery life feels much improved over my N1. I had to buy a second battery for my N1 "just in case" but the NS seems to last longer.
The device *feels* faster, but since Quadrant doesn't run on Gingerbread yet, i can't say how much faster it actually is.
Multitouch is SOOOO nice to have. Nuff said. Still, I use Swype for my keyboard choice 99% of the time, so the only thing I actually use MT for is Google Maps to rotate maps in 3D mode just to show off to my coworkers who have iPhones or PalmOS devices.
Not a fan of the headphone port in the bottom right corner of the device though. Understandable given how they designed the battery cover, but it'll make a car dock hard to find... Battery cover is a pain to remove, and feels flimsy, but it's sturdy enough.
All that said, there's one thing that still has me considering returning my Nexus S ... lack of an Android-recognized car dock. I bought two N1 car docks, one for my car (duh) and one to stick to the base of one of my LCD monitors at work to use as a landscape clock/Pandora/TweetDeck/whatever.
Hope all of that helps. Ping me if you have other questions about the device that I can share.
netninja, i read a tweet the other day that Cyanogen has a CM7 Alpha that works on the Nexus S, so *somebody* on the team has one.