Ahhh... that is a UI issue... getting user satisfaction on a UI design is a tough thing indeed.
You see... I've yet to see a forum software I've liked all aspects to.
Delphi... chases users away with its multilevel memberships, and limited customizing. Best all around though. Does not support BBCode, but a hybrid HTML varient. Search depends upon membership level.
Beehive... Per others, slightly more flexible and better speed than MXBoards, but a royal pain to maintain. But has advantages in left side menus (I find it infinately easier to navigate that type than here), and file attachments in posts and PM's. it also accurately tracks which posts have been viewed or not, and if the person you direct a post to has read it or not. Does not support BBCode, but a hybrid HTML varient. Search works and is unlimited.
VBullitin... a bit sparce, supports BBCode, search only seems to work so-so for me when I've tried it. Have not tried PM's, dislike navigation style
InvisionFree... Post edit screen is very annoying, dislike navigation style, no file attachments allowed (at least on the ones I go to). No way to direct a post in a thread to a specific member, no way to know if specific member has read post. Also has habit of sometimes resetting the threads visited function, if close browser before seeing all the new threads. Supports BBCode. Additionally, it allows only one skin in the forum, and no default provided if the site owner overrides it. This is an issue for me with the Be Your True Mind forum, as I hate that scheme. Search function is a utter failure.
PHP... Of the ones with no left side navigation, I like this one best. Supports BBCode. Does not support attachments in the ones I go to. Search is iffy. Skins suck, at least at fukufics, since it breeaks most of my navigation and scroll keys in IE. Lukily, unlike Invision, it allows the system default to still work if you override the default. It has the dreadful problem of resetting the threads viewed indicators every time the site is visited with a different browser window. Feels smoothest, but has occasional downtimes, unceretain if that is due to the server or software though.