One of the most useful addons, since lots of sites worth watching don't have RSS. However, needs a few improvements:
1) Handling of errors - perhaps an option to ignore a subset. redirects usually should be ignored; 404 usually counts as a change... but seriously - treating 101 ("network unreachable") as "page changed"?
2) Ignoring reading/writing of cookies, per page. Sometimes allowing e.g. auto-login or theme choice is a good thing, sometimes not. E.g.: I only want to check a forum for new threads, which can be done anonymously; but if logged in, "new posts here" variable will be reset by SiteDelta's visit, so doing it on a schedule is a bad idea. Same goes for any other "changes after your last visit" pages. It would be even better to interact with cache, but that's overcomplicated.
3) Check the pages BEFORE other addons, such as e.g. Greasemonkey scripts. So that e.g. Webcomic Reader or other custom layout script toggled on/off won't count as a change.
4) Allow watch for elements other than html body or visible divisions - sometimes changes in html head or script constants are also need monitoring, and sometimes something that can't be pointed with a mouse needs ignoring.
5) Controlled minimum delay after the browser starts. No point to slow down FF startup when it's used to look up a help file or something.