http://www.hattrick.org/Help/Supporter/Having access to federations is probably the best feature, followed closely by access to tournaments (private leagues). The bookmarks also come in pretty handy when searching for guys on the transfer list.
The link above gives you the full least of features, I've has supporter the entire time I've been playing, so I tend to forget what's a supporter feature and what's not.
Also, if you use Firefox (or Chrome as well I think), install the Foxtrick add-on, it's got some really handy features (which often results in a lot of people forgetting whether they are Foxtrick features or actual HT features!)
One month of supporter is around $6, so not too expensive. It also removes all of the ad banners, making the site run a bit faster, too.