HTML (Especially XHTML) is something that forum owners have wanted more and more of from their web forums, especially with CSSed elements (for colors/styles).
Additionally a MySQL/PHP backend is the best option at this time for forums...
Having "plugin" support (for features and the like) is also a good idea as it will allow users to add in features without having to hack existing code.
A few things to steer away from would be Zend or IonCube encoding, if users cannot modify the source for their own needs (features and/or styles) they will not use their product, a few forum solutions tried to go this route and almost ended up dying out completely.
This is Gizmo btw.
*Hell hath no fury like a womens anger and damn be the fool who gets in her way*
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