Personally, I'm always happy if someone is reading my stuff!
On a more serious note, though, I usually try to start my critiques with things I liked best about the story and then go into the things I thought could have used improvement.
If you've got a whole series of typos or other small corrections, you might email the author rather than present a detailed nit-pick in the comment section.
If you can see that the author is actively writing and posting to the archive (stories posted since 2007, preferably some in 2010 or 2011) - and you really like their work otherwise - you could always offer your services as a beta-reader. Most of us could always use another set of eyes.
Hope this helps!
Julie K.