I believe the 'Share' button should be replaced with the 'Like' button, because the word 'Like' is more attractive, so it will make more people click on it and by that publish forum-posts in Facebook. Good Luck!
In the Member List (http://community.invisionpower.com/members) there is a bar for letters (A,B,C...) to begin with.
Because not all of your customers using English, I think you should allow add other letters in other languages (in the Admin CP).
Note: If you add this option, I think you can order the letters automatically by ASCII char indexes.
As for the discussion opened here, I think both options for quick reply are not good (simple textarea - and - full editor).
I think a "minified" rich-text-editor, with one line of buttons only for bold/italic/image/link/emoticons - is much better.
And another thing... I think after the title "Topic Tags" you should add a grey text with the word "keywords", like:
I think the disappearing of the "Display Name" field from the registration form is a mistake - because one of the good things with display names is that the others users don't know your login name and if some user have a weak password, hackers can't login because they don't know the login name. Now, while you changing your display name, the change is written in the "Display Name History" in the profile, and now everyone can know your login name.
For your attention.