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.
I've noticed I haven't said my opinion on the anonymous posting......it sounds like a good idea...maybe there could be a little tick box near the thingy bits when making the new thread saying that you'd like to be anonymous...the only problem with it being that we are all used to each other's writing...so if you saw an anonymous post in there with more punctuation than you can shake a stick at, you'll instantly know it's mine etc...