If my weakness happened to be grammar and spelling I wouldn't be a beta. Of course that's important and some of you are probably wondering what else matters, but sadly I do have several undesirable flaws that can be worse than poor punctuation.
The first would be tenses. I absolutely stink at tenses, and in my defence they hardly matter. It's okay to change tenses slightly, sneakily, as long as it doesn't interrupt the flow of the overal writing. That's my opinion though.
The second would be paragraphing. I know paragraphs change when the topic changes, but I'm horrible at detecting topic changes in general. This is probably my fault for trying to blend everything together smoothly for the "flow" I discuss so often. I hate really big paragraphs though so if anything I'll probably make you break up your paragraphs.
The third would be that I'm comma happy. I use commas at least three times more often than periods and if not for the semicolon, I'd probably be in trouble. I've also noticed I went easy on the commas while writing this, which makes me kind of happy, though I'm itching to add commas in at least ten places.
Fourth would be my use of the apostrophe, I even just added "the" to avoid making apostrophe plural. Just now, I wasn't sure if using an apostrophe at the end of apostrophe would be appropriate. Insert head explosion here. Don't worry too much about my use of the apostrophe though. While betareading your work I'll definitely have my handy grammar book glued to my side, as well as a realiable dictionary and thesaurus.
Since it'll be your writing I'm editing I'll be on extra high alert; which probably means I'll be a bit irritable at times.