Been to Wembley a few times for NFL games, but this was the first (proper) football match I've been to there, and I agree with a fair bit of what's been said.
There's a fair gap between the stands and the pitch. Not massive, but definitely noticeable. The view is great, but the acoustics suck. Seemed impossible to get any songs going for long because the blocks couldn't seem to hear and/or stay in sync with each other.
The crowd itself didn't seem too bad where I was, down the front of 118 in the corner. People seemed to try to join in and be up for making some noise. Everyone also stayed standing (I'm not that bothered personally, prefer standing, but I'll happily sit if necessary - didn't seem to be necessary). I could hear Cardiff at times, but I couldn't really make out what they were singing where I was - we seemed louder, apart from a lull in the middle, but being at the Liverpool end you'd expect that really.
Watching it on the telly today, it came across as pretty decent - better than it did in the ground.
Overall, great views, bad acoustics, but I enjoyed it.