As for a one-man-army (solo developer), GM48 happens in two phases.
The first phase feels like game-design poker. It's all about making choices, about trying to stay in the balance between "What I want to do" and "What I can do in 48 hours" while picking features that fulfill the idea of the game the most.
The second phase is actually a trial. It's all about applying all acquired experience of the GM:S, inventing solutions to new problems and hacking things through while sustaining time and real-life.
Altogether, that's a great opportunity to experiment with a game design, due to the new theme each round and fast and honest feedback.
Wanna design game from topic to features, or in opposite way? Wanna express your current mood, or practice in designing core loops, or to explore new grounds and combine hardly fitting features?
It's all possible, and all welcome. And when you're ready, there's next level for you - competing for the first place in ratings, applying everything you've learned.