I still maintain (as do lots of others) that the best way to come to grips with writing is to read. A lot. All the time. It's amazing how much you pick up without realising it. You might not know the rules of grammar or punctuation or spelling or be able to rattle them off by heart (I certainly can't) but if you read, things will start to look 'right' or 'wrong' to you. You'll get a feel for it.
I tend to critique in a narrow band of what I write and read. Horror, SF, and military are what I tend to write, and I'll critique crime and a bit of historical, since those are within my interests. I'm allergic to fantasy, read it a lot when i was younger but somewhere in the last twenty years or so I just started to retch at the mention of elves or mages or "Lord Nanchara". I don't think I've ever read a romance story, I should try some day, but having said that, I wouldn't feel competent to critique something that I know nothing of the conventions of. As far as SPAG errors, we all make mistakes here and there, but if you're a native speaker and your work is riddled with them, I'm just going to hit the back button. At this end of the screeen I'm in no position to tell where the fault lies, but an inability to use the basics of English is not something I'm willing to help with.
Typically I read genres that I have an interest in. I only critique items I read and I have items of comparison in topics I'm familiar with. Simple.
I honestly have no preference as to what genre I choose to critique. Writing is writing. If I like it, I like it.
