I think you hit the nail on the head about it providing a limited environment but with modern features. It captures the feeling of what it may have been like to make a Game Boy Color game but without the headache of actually trying to get that set up. Similar to how Arduino made programming microcontrollers easy (they were founded because the founders found it annoying to do).

Besides the features others have mentioned, I would say simplicity, how easy it is to get started, and the community/BBS which promotes easy sharing of work. Splore is a great tool and with the web site makes it dead simple to explore and learn about new games.