Cuba has extremely friendly people, the staff are usually very nice but due to the relaxed nature of the Cuban people you'll find service isn't what you'd expect in say Mexico; not because they don't care but its just part of their culture. Cubans are so awesome, you'll forgive them for not rushing to bring you your drink or food.
Yes food is the only real complaint about Cuba, the embargo makes for problems and if food is a big reason for your travels avoid Cuba.
Dominican, especially Punta Cana with its amazing beaches is a great destination, however you can end up with somewhat hit/miss service, I don't agree that you'll find better service in these places over mexico. I varies a bit in all countries.
Mexico is awesome, if you're able to afford it go to the mayan riviera , Cancun is a bit more of a party place depending where you are at, and the beaches are eroded in most places to a point of being rather sad.. I'd highly recommend the Barcelo Maya, and the newer part if you can afford it.. The resort is HUGE, has some of the best beach in the mayan and because of its size its hard to be bored if your kids are with you, lots of pools, bars, restaurants and plenty of kid activities. This hotel even has a long standing fan home page that has tons of info:
http://www.unofficial-barcelo-maya.com/site/
As far as what to expect, 5 stars in Mexico is something you can be pretty confident in, Cuba you tend to knock 1 star off on the ratings you see in brochures, online ratings tend to skew to being too critical of everything so you might want to remember than when reading online comments, usually not a great indicator unless its overwhelmingly negative.
Dominican I'd suggest in order Punta Cana as the best place to go (Barcelo beach/golf/garden/palace resort is amazing, HUGE and lots to do, great for families,) Puerto Plata next, usually inexpensive in comparison, the beaches aren't as nice but its a fun place for sure.. The last is the long stretch of beach near the capital, Santiago.. It tends to attract alot of Europeans and honestly while there are some nice gems in that stretch, most of it isn't worth going to. Since I brought up Europeans, I found in my trips to Dominican that you'll find a ton of them, which is nothing like Mexico where you usually find mostly english speaking guests.
As for all my clients in the old days I always said, the only way to be a good traveller is to be patient, not get upset over little things like not enough towels, leaky AC or things that you sometimes run into. Some people are way to picky and lack patience for traveling to the 2nd and 3rd world; so go to have fun and don't go looking for reasons to get upset.