I agree with Stuart’s points,
The main thing I use the site for is to ask for help and share stuff so the forum element is central. I think making the discussions quicker and easier to navigate would be great.
The groups I find a useful way of organising and storing content so I can quickly refer back to it when needed. Before talkphysics I often discovered great resources but had forgotten about them when planning a relevant lesson months later. I find talkphysics superior to del.ici.ous/google bookmarks as simply reading and taking part in discussions is a great memory aid (aswell as a place where people add even better stuff that you didn’t know about).
I like ability to embed videos and images is v.good and it would be a shame to lose this if you switch to a more basic forum structure however I think a robust discussion system (easily to navigate and organise) is what’s most important to me.
I think the ability to edit messages would be useful, if only for 10-20mins window after posted.
I do think direct messages are useful though if you just want to privately thank someone e.t.c.
I found the CPD sessions pretty useful.
second the point about logging in, my web browser doesn’t save my usrname and password for your log in screen some reason.
some form of calender of physics/IoP events (e.g. physics update)
summary of current grants available for physics teaching/clubs
summary of national competitions e.t.c. that students can enter
‘like it’ buttons like on youtube, just say a quick thanks for a really helpful comment e.t.c.