Advantages: nearby pools/clubhouse/playgrounds, etc.
I've seen a lot of posts on DCUM in general about joining pools with long waiting lists, high fees, inconvenient hours, etc. It makes me glad that we have a neighborhood pool so I don't need to worry about the waiting list, or choosing which one to join, etc.
Disadvantages: HOA fees, some HOAs are very strict about things you can do to your house (what you can plant, what colors you can't paint the outside, etc). Personally, I'm just not a home improvement/home decorating type.
Personally, I don't spend a lot of time/money/energy thinking of creative ways to make my home look nicer--I maintain it to normal standards of course, but I really don't try to improve on anything and really am ok with "cookie cutter" houses. So HOAs work fine for me--I know what's expected of me, in terms of maintenance, and as long as I follow that, I can be sure that my neighbors do the same so their are no eyesores, etc.
I can see where if someone was really into their home, making fabulous, "stand out" gardens, etc. then being told what they can/can't do would be difficult.