vB3.5 is mostly an architectural change. Many hacks will not have to be reinstalled for each 3.5.x because of a new hack hooks system, and hacks and add-on software will be able to do things such as generate a post with a single function call.

User level changes include AJAX stuff, which will allow you to change thread titles inline from forumdisplay (before time expires, if you're not a mod), and to quick reply without a page load; moderators will be able to do a lot of moderation stuff with fewer page loads. Also, vB3.5 brings persistant read marking, so you can check the forums even if you only have a minute or two and come back later and pick up where you left off.