My husband and I have recently been talking about this quite a bit and are pretty set on moving back to the US in a few years. We have decided to have a baby here first in order to take advantage of the maternity leave, which is hands down better than the US, but our quality of life and standard of living is just not as good here.
I had excellent health insurance at my old job and have cotinually been denied referrals under the NHS to continue getting care that I had been receiving in the US. As it stands now, I am at risk of developing skin cancer because the NHS doesn't believe in preventative care, whereas in the US anything slightly abnormal was treated immediately.
My career prospects are much better in the US and I have found people in my field here in the UK to be downright nasty. I think I might be able to live with less money but can't really get past being miserable in my work life for the rest of my life when I adored what I was doing in the US.
This doesn't apply to everyone, but I actually had more vacation time in the US than on the UK when you add in holidays. My husband has slightly more here, though he can't really use it all because no one in his field takes theirs and he's expected to constantly be in the lab, even on weekends.
We had better access to public transportation where we lived in the US and our commute time in the UK had more than doubled, with really awkward train and bus routes that force us to walk for an hour at a time most of the time. This is fine for now, but there's no way I see us making that work with children!
The other big issue is affordability, which has been covered enough already by others. I don't think our issues apply to everyone in the UK, but I was much happier in the US and find the UK doesn't have enough benefits to outweigh all the negatives. But we were living a car-free, fast food-free, support local products sort of lifestyle to begin with, so the lifestyle change that a lot of people have mentioned wasn't anything new to us. Nothing will change about our lifestyle when we return to the US other than what I've mentioned above.