I saw your reviews/comments on a few stories today, and I was happy. I just want people to come out of their shells more. I learned so much about writing from reviews. I finally had to give reasons why people did some things, and just taking the time to explain a few aspects can greatly increase your understanding of them. Then I see people complain about wanting to improve their writing, but not doing reviews.
Anyway, I stunk at reviewing at first. I had a few people correct me the first couple times I did it, but I stuck with it. Eventually I got better, but I was mainly sticking with grammar, as my knowledge about description and characters aren't the best. I remember I reviewed this one fic, only to have someone else come in and completely blow my review out of the water. It's not a competition, but it made me realize how much I still had to learn.
If you're worried about reviewing grammar, don't. Stick to reviewing plotlines and characters, since that's what you're really good at. Gradually move your way into grammar, and if you're unsure of something, just leave it alone. Reviews take effort and practice, just like writing does. If you don't like your reviews now, keep working at it. I promise they will improve.