On some sites ( Reenlist for example) when you hover over the title, the first few lines of the post are displayed, which makes it very easy and fast to decide if it is of interest, as titles are not always descriptiive.
Might require an entirely new forum engine mind, I'll admit it can be handy at times on those sites that I've used it on (though those are few and far between).
I can see how this would be advantageous, but would require some serious optimisation of our our data to get this to work. So not something we have time to do at the moment... will add it to the list though.
