Personally I like both short and long comments. By all means point out mistakes if you spot them (even beta-readers are human and fallible!) - but when I'm commenting on a fic, I always start by saying something positive, before I point out any errors - and I usually say something along the lines of "Sorry to mention" or "My inner beta-reader/Brit-picker notes..." before pointing out the errors.

(I once had a review which started in with someone's very picky comments before saying anything nice, and I felt like the commenter had slapped me - also she was American and I'm British, so half her points weren't even valid because the two versions of English have different styles, etc.!)