I think the most important thing is to be able to add our own HTML directly in the page - because that way we aren't limited in anyway - we can literally do ANYTHING we can think of.
But the specific thing I constantly want to do (which at work I do with divs because using twiki - but at home can't) - is split a page into two or three columns. Basically because I constantly have sets of lists - it doesn't make sense to put them in one table, but also looks terrible with just table after table only occupying the left third of the screen, forcing the viewer to scroll while the right 2/3rds are completely blank.
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100% agreed. XHTML is the way to go. This would solve many, many formatting issues people are having.