I absolutely loved my Windstar trip to Costa Rica. We went Feb/March 2010. The Windstar was really beautiful. It was laid back, and the service was excellent. Average age of the passengers was about 50, but there were couples from 30's to 70's on board. Most people on board are couples with the exception of a very few people traveling with a friend. It is an active trip, and there is no elevator to go from floor to floor. Cabins are all the same with the exception of an "owner's suite". The difference in B cabins versus A cabins is that B cabins are on the lower floor. You will have to climb and extra flight of stairs to get to the public areas. This wasn't a big deal at all as it helps burn off calories from the food. It was an intimate group of about 100 people. We got to know most of the people on board, and it was a very friendly group. There were no children on the trip.
Embarkation was easy. We were on the boat with our paperwork filled out in less than 20 minutes. We were some of the first on board, and there was a snack (sandwiches, fruit, chips & salsa) on the pool deck. Dinners were served in the dining room or Candles on the deck every night. Most people were casually dressed for dinner. They refer to it as "casual elegance". Most men wear polos and khaki pants and women wear sundresses or pants. After the first night, I left my dressy shoes in the room and wore my flip flops with my sundresses. The only restrictions for the dining room are no jeans, sneakers, shorts, or T-Shirts. There were a few people in jeans towards the end of the cruise.
You will need plenty of clothes. It is very hot in Costa Rica in early March. I wished that I had packed more T-shirts, shorts/capri pants, and underwear because I had to get laundry done. Some people got the $89 for the week laundry, which wasn't a bad deal if you wanted to pack lighter. I took at least two showers every day, and on a few days I took three. Leave your jeans at home except for traveling, as it is far too hot to wear jeans.
All of the cabins were about the same except for one "owner's suite". I looked in when it was being cleaned. It had a separate living room area. I'm not sure it would have been worth the money.
The difference between an A and a B cabin was that B was on the lower floor. You have to climb an extra flight of stairs to get to the main levels. I didn't find that was an issue.