It depends almost entirely on the specific crowd for me (oh, and sometimes my mood).
I like party type crowds when:
There are at least a handful of fairly close friends attending
The people I don't know have a high probability of being interesting/quirky/smart
Stimulating conversation is bound to occur
I dislike party type crowds when:
I don't know many people
When most people are overly concerned with their image, not looking for meaningful conversation
The (bad) music is so loud as to drown out any attempt at conversation
^although sometimes I enjoy these types when I can sit back at a distance and think
I like concert type crowds when:
I have at least one close friend accompanying me
I really love the band that's playing
Some douchebag near me doesn't ruin my experience by being drunk and/or stupid (don't yell "Freebird" dipshit)... usually in this case I just move to another spot
I dislike concert type crowds when:
The crowd is too big
The likelihood that obnoxious drunks are going to be screaming and generally being idiotic is fairly high
The likelihood that a good chuck of the audience doesn't really care about the music is fairly high
(These usually occur when bands are pretty popular, worse if bands are VERY popular)
So basically, all the concerts by musicians I like should be safe... at least for awhile (probably forever, judging by the (lack of) mental capacity of the masses to (not) appreciate complex/unique music).
On another note, my boyfriend dragged me to the gym the other day just to see if I would like it. The first night, probably around 8:30-9 pm, I enjoyed it (probably only 5 or 6 people in the gym). It was quiet, I was able to relax and clear my mind. When we went at 5:30 later that week, it was a miserable experience... way too many people around, who were way too concerned with making appearances and meeting people for my taste... I just felt very out of place. I couldn't concentrate on relaxing at ALL. Made me very uncomfortable, and I found it odd that I hadn't really had an experience like that before. While I dislike some crowds, I never had really felt uncomfortable/fidgety/anxious before that.
I enjoy airports, stores/malls, busy parks, busy libraries, as long as I am not inconvenienced by the amount of people. Headphones help immensely.