Nice. (Almost) full implementation of css replacing most images. I have done that somewhere.
The expand/shrink or up/down icon, need not be that big. It's ugly anyway. Try a smaller one. I would also sugest css icon based on html code (I can't remember where did I see one but you can search it of course).
A tooltip replacing poster's info is nice. I have put up this idea somewhere before (I think may not be here but in bloc's forum somewhere). But preferably, a simple avatar/picture is enough. Too much details shown are not necessary but may be left as an option whether to show or not to show.
The design of boardindex should be fully changed or at least an option whether to use that or a new homepage for SMF. I think bloc has some idea on that. It should, IMHO, show more details like topic, topic starter with smaller avatar/picture, in which board and few latest responses to it. A more like forum member's activity page to be precise. A member can choose whether to show all activity in the forum or just his buddy/friends plus recent or active (recently replied / commented) activity.
I haven't register yet, so I may not see other features. But do consider what people have long asked from SMF that is topic/post like system. Karma system on the other hand should be just a mod.
A mention/tag system and activity streaming center as opposed to information center? (monotiz has done these), a new and better profile page (like simplenetwork mod?), are all to be considered.
Overall, it looks just like SMF in SMF ways. I am not really sure whether I like or not but, it do has some improvements though may be not a major one and may be not to the satisfaction of all. Besides, who can satisfy everybody.
I wish the team, good luck and all the best.