i have not read though the wiki in great detail but several questions that come up often:
How do I upgrade my smf to xxx version?
How do I _down_grade my smf install to 1.1.*?
Why is my database backup not complete? this is when doing a backup from the admin panel not from phpmyadmin
Why do I get permission errors when trying to install a package? e.g. missing the temp folder in packages
Often users are asking questions about mods they have installed in the smf specific boards and not in their support threads, maybe a wiki page explaining why smf does not directly support the mod could be useful.
I have seen several people attempt to attach mods they have modified to a post, then to have to be told that this is not allowed, a wiki page on this policy could be helpful... I would suggest such a page be locked from general editing though.
Several users recently have posted about crashed database tables, a wiki page explaining how to repair such would be helpful.
A page on how to move an smf install from one host to another, or from one folder to another and running repair_settings.php
Possibly a general [locked] page on what to consider when giving out your admin/ftp/cpanel login information to anyone to assist in getting issues resolved.
there might be already a page on how to enable/change version emulation for smf 2.0 then reset it back to normal in order to install mods that are not already updated.
instructions on where that check box is for installing mods on other themes, or at least attempting to install it
what this mod does http://custom.simplemachines.org/mods/index.php?mod=2054
and how to properly use it when requested to use it and post the new error logs.
I have seen several issues with the pretty urls mod when people change servers or move their install, maybe that issue could make the wiki faqs