Describe a game experience that changed the way you play.
I've seen that quite a few people have been tripped up by this question. Mostly because they already answered this when they gave their response to the 7th Day with its question about impactful sessions.
So, if I'm going to answer it differently, I think that one of the experiences that really opened my eyes to the potential of play styles was the first convention I attended back in 1994 or 95. I've tried looking it up, but since the convention collapsed pre-2000, there isn't a lot of online evidence regarding its existence.
In my home groups, I was always the default GM, so I attended the convention in the hope that I'd get the chance to play. It didn't quite work out that way, but it did work out by showing me numerous styles of GMing and learning that even the "professionals" were often making it up as they went...in fact the most professional GMs and the ones with the games in highest demand were the ones who ad-libbed and played reactionary games rather than trying to lead their players through a story.
...but that's more about which experience changed the way I thought about being a GM.
For the experience regarding play, I think the Raven's Nest game was the biggest eye-opener. This was a game incorporating miniatures and live-action in the manner of a Braunstein. There weren't enough GMs to keep everyone occupied during this game, so a lot of it ended up being a case of players telling their own stories and interacting with each other until dramatic moments when a GM finally finished with one group of players and potentially came to ours. The big lesson here was that you didn't need a GM to have immersive fun. Sometimes they just got in the way.