|Here are a few things I think you should keep in mind:
1. Don't ever mark a note as useful unless you actually read the entire note and really think it's useful. You aren't helping anyone if you're checking "useful" just to boost someone's points. I'm not saying that you ARE doing this, I just want to be clear about it. :)
2. If you don't have time to read the entire note, just skim it. Or, maybe just read the notes written by photographers who you already know write really good notes. And you're right -- reading the notes can be a lot of fun. Just the other day, Luc posted a picture of papayas and actually gave recipies for papayas in his notes!
3. Above all, do what makes you happy. You say you sometimes don't have time for reading notes after writing the maximum of 20 critiques per day, which I find troubling. That makes it sound as though you're at work, struggling to make deadlines. TN is supposed to fun! I think you should cut your critques down to 10 per day. That way, you would have more than enough time to read notes, AND you'd also have enough time to write even longer critiques.
And to answer your last question, I think the photo is of course the most important aspect of a post. Sometimes notes are funny, sometimes they're facinating, and sometimes (rarely) they're useless. The mission is, after all, to learn more about nature through PHOTOGRAPHY, not notes. :) Hope this helps!