One of the most important things at Mandalay Bay for me is the fragrance--I get deathly ill from cigarette smoke, and Las Vegas allows smoking pretty much everywhere.
Mandalay Bay pipes in some sort of exotic, tropical fragrance into the air. It is very refreshing and lovely. The casino itself is very brightly lit, and you can barely smell the smoke, even if you're sitting in the same row as someone smoking, you can't really smell it.
I also highly recommend at least one of the restaurants inside--House of Blues was absolutely wonderful to myself and a friend, who met there for breakfast. They do have a "Gospel Buffet Brunch" for which you need to buy tickets in advance. The line gets very very long, but you can just go in for a regular breakfast if you like--the staff is very sweet and helpful, and attentive (but not in an obsequous way). We had a great breakfast with amazing service.
Mandalay Bay also has a lovely wave pool, and a lazy river--during summer, the river can get quite popular with kids , so you might be better off tackling it early in the morning, or late int he evening.
I can't speak tot he rooms, but if the service towards the guests is anything like the service to random passerby, I don't think you can miss.
There is also a monorail from Manadaly to the Luxor and the Excalibur--it's a great way to get down the Strip towards more central Las Vegas, and it's free. Watch their website for specials on rooms.