Apparantly my experience was a little different...
I went three years ago on a family outing to celebrate my mothers successful kidney transplant....It was my mother, father, my wife, 5 year old son, 2 year old son, and myself. We had a great time, and did quite a bit....Our hotel was nice, and it was an all inclusive, so my only complaint was that there was constantly stupid ass drunk teenagers running around....
If you stay at a nice hotel, you can count on a few things...first, all the water will pretty much be bottled. Secondly, as someone else mentioned, the conceirge will make sure you stay away from any undesirable stuff...I went into downtown Cancun in a taxi to the Walmart to get groceries and had no problems....the taxi driver waited on me and didnt charge me to wait...you just have to make sure you know how much it will cost BEFORE you get into the cab (again, thats something the hotel will tell you). My wife and I went to Coco Bongos one night, and while we were waiting for it to open up, we killed some time by having a taxi driver drive us to a strip club. That was the only time I got nervous...It took about fifteen minutes to get there, and it was deep into "Mexico". I got real nervous because Im 6'3 and about 310#, and it seemed like every single Mexican guy I saw was making comments about me being "Rambo" and how I must really be able to kick ass....I was having trouble figuring out if it was sarcasm or awe....At any rate, the strip club was exactly what you would expect....chics who werent good looking, but would take you upstairs to **** for about $300. Obviously I passed since I was with my wife...
All in all, if you use common sense you'll have a good trip....I highly recommend Coco Bongo, and beyond that, take one of the "day trips" you'll see advertised everywhere...