It depends on what part of the ground you are in. I'm in the Scoreboard End, to the left of the goal near the North Stand. Everyone in our section is stood up as the stewards there allow it. Same with the Stretford End. Pretty much everyone in my section is vocal, and you usually see the same faces so everyone usually knows each other.
The problem is when someone you don't know is sat near you and they are too busy taking pictures and gawping at the stadium to join in with the rest of us. You get this a lot in European and Cup games as a lot of people now opt out of the cup scheme. Wolfsburg was one recently. I'm stood there and a bloke with a little girl sat directly in front of me, and in the seats next to me on the right there's an Irish fella with his two kids. Nothing wrong with that at all as I've taken my son to a few games before now, but I'm then a bit concious about not swearing or being overly aggressive and the like. This is what happens though I suppose when tickets are on open sale. You will get people you don't know near you and then you are then not wanting to cause offence through your actions.
Not to point fingers, but I had this when I had a season ticket in the North Stand a few years back. Everyone nearby was sat down, no-one bothering to join in with the chants that were coming out of other sections of the crowd, except for an occasional "UNITED!" here or there. I even had some Asian lad tap me on the shoulder to tell me to sit down as he couldn't see once. While we had a freekick just outside of the area. To be honest, that was what made me move from the North Stand. If I wanted to sit in silence to watch football, I'd have stayed at home and got it on Sky.
Yes, the daytrippers don't help, but neither do the moaners either. The Sunderland game this season is a case in point. United get a corner in front of our section and instead of the applause that the corner *** gets, I heard a few lads a few rows back screaming "make sure you get it into the f***in' area Nani, you useless c***...). Now how does that help anyone? Nani frustrates me, but by God I'll applaud the lad if he's taking a corner right in front of me wearing the shirt of the club I've supported through thick and thin since I was a nipper.
Don't forget about all the reds that have knocked it on the head as well, either to watch it in the boozer instead or to go to F.C.
This added to the all seated stadiums, and some of the over the top stewards means that the atmosphere isn't as good as it used to be. But then United, as a club, aren't that bothered really as long as the stadium is full, they will happily take the money.
It's just one of them things though. You will get a great atmosphere at a big game as it's easier to get excited by playing a Barcelona or a hated rival like the scousers than it is to be playing someone like Fulham or Blackburn.
It's not just a United thing either. Even after all the media pandering to Liverpool with the Kop being "their 12th man" I've seen them on the telly a few times and on occasions you can hear the players shouting to each other. Now look at how they were when they played their cup final recently against United. It's the big game mentality. Every club has it, and remember the fans of the likes of Blackburn and Stoke, playing United at Old Trafford is their cup final and as such their fans treat it like that.
In summary, as I have wasted far too much of works time on this post already, I think the atmosphere is generally good, although that could just be that I'm in a decent vocal end.