Mostly I've just witnessed BJJ evolve/change.
I've noticed things are different stylistically than when I first started. You have a lot more white belts playing open guard, DLR, berimbolo, and stuff. Back when I was a white belt it was a lot of closed guard and take downs. Maybe some DLR thrown in here and there. BJJ seemed more applicable to self defense to me.
I've also noticed that people in general nowadays seem a lot less tough and more prone to complain about getting smashed, crossfaced, other guys using strength, etc. Also, people complaining about getting tapped to many times or thinking more experienced guys should go easy on them.
When I was a white belt and blue belt I just took ass whippings and kept my mouth shut. Nobody went easy on me, because I was little. Nobody expected anything from anybody. You slapped hands and rolled. If you got your butt handed to you, oh well. On to the next person. You accepted it and kept a positive attitude.
I guess one of the main things that has changed for me is my opinion on belts. I feel like I've become a bit jaded. Don't get me wrong, it feels REALLY good to get promoted and be recognized for all your hard work, but I don't put very much stock into them anymore like I used to.
Even with all the ups and downs, in the end it's something I still love and feel passionately about.