let me preface this with the fact that this is a loaded question as I have made vast imptovements to my 2011, and will be attempting the same on the 2012s, but for now only know the stock 2012. Not to mention the sheer maximum power difference between the 2012 and 2011 models.
That being said, here is my experience with the two :
My 2011 was great when I first got it, I never expected to wanting to go as fast as it did right out of Hollywood Electrics, 68mph, but as time went on, and I started riding the canyons and visiting places like Newcombs Ranch on HWY 2 way up in the mountains, I realised I wanted to go faster. Hollywood electrics helped me out by giving me all the information I needed to get started tweaking my bike. I increased the accelleration and top speed, and was pleased with handling until I started scraping hard parts. The bike even with my modifications is a solid, reliable machine that I can depend on ( and is for sale so I can get a 2012... wife's rules saldy I must sell ). It performed extremely well on the track and off, and really impressed petrol bike riders and track instructors. The only down side was as expected the limitations of how far I could go with the battery at those speeds, 20-25 miles, more than enough for each session on the track, but cutting it close on my commute and crazy trips into the mountains where I often had clench my buttox hoping I didn't run out of charge before I got to Newcombs because there were NO PLACES to charge for long stretches of mountains.
The 2012s I have ridden are phenomenal machines, and I like them very much. I have topped out a stock 2012 Zero S at 98 mph on a cold night. And been able to run on the track with equivalent speeds and performance to the 250s. The bike really handles my crazy corning speeds and late entries outstanding. The overall handling with the adjusted rake angle is greatly improved on the track, it is subtle on the street, but on the track it is similar to cutting butter straight from the fridge ( the 2011 ), and cutting butter that has been warmed up just a little, but not too much ( the 2012 ). I was able to go into corners at speeds faster than riders of my skill set on petrol bikes, maybe I shouldn't have though since I did run wide once or twice, and fly off the track between 65 and 75 mph lol. I really do wish the bike had better acceleration from the factory, but am satisfied to know that I will be working on improving that myself. Also on the track the controller does need a better cooling system. I think the motor is fine, but the controller can over heat and lead to some confusing situations. Again, only on the track so most people will never see this, and it is relatively simple to fix, IMHO, so my racing partner and I with some help from Hollywood Electrics and Phidgets ( one of our sponsors ) will be designing a racing add on for the 2012 that will cool the controller better and make this a bike that can ride with and even beat 250s on many tracks.
All in all, I love both bikes, and they both have different purposes, while my 2011 is fun on the track, it is more like a vintage bike, and if I don't sell it so I can get a 2012, that is probably the class I will be racing in on it. And the 2012 is like a 250 SuperStock bike, performing well on and off the track.