corenting already took part of it but several youtube tags are iffy at best (some of the really old ones like the youtube video ID options had been broken before the upgrade though). Also many of us had got used to the youtube autoparser but that is not so bad as it just leaves a URL, for a nice example of the mix have a poke around through the
My main two problems would be the topic tags are missing- if I ever had runs of features or threads likely to be of similar topic (release posts to games that have hype/discussion threads and such) I would use them as a search.
Second and it probably serves me right for using a deprecated feature back on IPB was that the title command ( [title:sometext] ) of the spoiler box no longer works.
Other than that the new forum is fast as anything which will certainly help me get used to the new skin (GBAtemp v3 was a bit slower than V2 although I eventually tweaked things which made it fast) and everything you are probably already aware of (
http://gbatemp.net/newgon/ gone, ask gbatemp links not working, the old HTML <area> stuff in the title bar being gone). I will probably have to get used to the new reply box (it keeps existing formatting which I sometimes dislike), but that is neither here nor there.
If I click the alerts button in a thread some of the text of the thread below pokes through. I will have to check that it is not my paranoid security settings doing that though.
I should also note threads I had followed as just a notification now email me instead, I do not mind and could get my preferred behaviour back after a bit of fiddling*, but in the interest of staying on the good side of spam filters and given the old forums had people that used the "follow every thread I post in" option that might not be ideal. How you play this is up to you although for myself I would not mind having to add email notification to various threads and as long as they were still followed it is surely all good so a site wide "regression" to site only notifications could work.
*I checked through the proper notifications and did not see anything, in the thread itself there was not a lot but unfollowing and refollowing gave me a checkbox to receive emails.
If we are going truly minor the likes counter in pre v4 threads says "1 people like this." for those posts that just one person has liked, but there are bigger problems than truly minor grammatical errors right now (I would probably think about changing the wording rather than coding up something that will spit out a different message as plurality demands).