I have been beta reader for years. Mostly for friends, but I did a couple projects for writers on AFF years ago. You tend to see your own mistakes in the writing of other people. As a writer, it's difficult not to compare your own writing skill with the one you're correcting, especially in the case where it feels like they're leagues above you, but that's an excellent opportunity to have a little of that skill rub off on you, right?
Betaing definitely made me more aware of conscious comma use and language barriers. It's tricky sometimes to see the difference between something that should be reworded and something that's simply stylistically different than your own. Recognising that difference, I think, made me more open-minded. Though I did go through a time where I took a lot of things apart that I probably could have relaxed about. On that note, betaing is also an excellent way to learn tact!