It isn't the new format - people get used to change. As Ash says bringing back treeview won't make much difference. People did have trouble with their logins after the change despite your sticky and some may have disappeared for good because of that.
I think moderating the spammers would help a little bit. There were a few more mods on the old board and stuff usually got dealt with quickly - takes days on here sometimes.
Other than that - perhaps there is a slight feeling of malaise? Not just Arsenal but football in general - the PL is just so much overkill these days...no real break, constant crap on social media and the tv. I dunno, perhaps I'm just getting old but the spark just isn't there any more and even if AW had broken the bank I don't think it would make bugger all difference.
Lastly, get rid of the sweary filter for ****s sake