Simply put, a relaxed and nice environment with great people and amazing diving.
I took my Advanced OWD and my girlfriend took her OWD certificate, then we went on 8 additional dives (that we hadn't planned on, but we just couldn't resist).
The instructors and dive masters have great personalities and are really passionate about teaching as well as diving.
They have hotel, small restaurant, sun beds and a dive center all in one.
They pick you up for free at the hotel. Then after the morning dive you go and relax by the pool, do some carbo-loading in the restaurant (relatively cheap), and just relax in good company.
The gear generally works like it should (which is the reason for the 5 stars when it comes to "Attention to safety and environment").
I was totally amazed by the overall experience we had and the wonderful people that work in Blue Planet Divers