For myself (all of which are features that had never occurred to me before buying the Touch but are probably something I'd mention to people asking for a recommendation):
1.) Highlight/notes/dictionary in sideloaded Epubs. This is my first ereader, but I have a fairly large selection of instructional or academic epubs from over the years (read on PC). I'd love to finally be able to make use of them, but it's not really the same when I can't highlight the important parts or make notes like with kepubs. As for the dictionary...something I never thought I'd use but I do all the time with my kepubs.
3.) Margins and line spacing options. Which I just read are on the way. Great.