You call that simple? Any tool where I have to markup my sheets using HTML and/or XML, I don't call "simple". I've a home brew tool as well and it goes for simplicity too, and then the only simplicity I care about: the parts I type. My sheets look like:
Title of the sheet
* Point I make.
* Another point.
<perl>
Statement.
Statement.
Statement.
</perl>
Text with C<< inline code >>.
E<< Emphasised text >>.
Of course, going for this kind of simplicity probably comes from my lazyness of postponing writing my presentations the night before I have to give them.