I've noticed JB is a bit less polished than the last revision of ICS. The voice dialer is a lot more irritating, it never understands either the person or the number (mobile, home, work) I said. ICS was perfect.
I also seem to get less battery life with JB. Maybe due to Google Now running? I don't actually find it that compelling, though it's a neat demo of technology. It's rarely needed.
I also find that Netflix and WatchESPN are worse. Sometimes they will only show a black screen, necessitating a reboot. Killing the app then restarting doesn't help, nor does turning off and on mobile data. Netflix also hiccups now when the quality changes. There's like a half second skip of audio when it happens. On cell data, that happens a reasonable amount, which is sort of annoying. That never happened on ICS, and I am almost never on wifi.