Certainly there's different ways to evaluate the system.
I'd like to add out of the box experience (Literally with the NB bundle you can go from download to "sample app" in 10 minutes). Even just the app server is trivial to install and get going.
Gradual complexity. The admin gui, command line gui, the config files, the documentation make the simple stuff simple and the difficult stuff possible. Combined with JEE 5, things mostly just work. Yet you have the depth of all of the configurations options to be able to take it where you want to go.
The packaging is nice and compact, and it doesn't expose its underlying architecture. Specifically, you don't have to be an expert on Glassfish and how its built or designed to get good value out of it. The others are certainly powerful and flexible architectures, but they expose that complexity to you as a user, and it affects the learning curve.
The domain structure is nice also, easy to upgrade the server and keep your configs in place. Important with something that moves quickly like GF.
Good support on the forums here too.
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