I, for one, strongly dislike thread preview hovers, as I always find them getting in the way while I'm trying to view other thread titles.
There are a number of other folks on the forum staff who feel the same way, which is likely why it has never been implemented.
You can't just NOT hover over a title while viewing thread titles? The preview just doesn't pop-up for no reason, you need to make a concerted effort...
PLUS...those pop-up are not immediate, they require a second or so of hovering...and they disappear in 5 seconds so even if you did leave your mouse hover over a title, your annoyance would be gone in a few seconds.
I really think the higher-ups should investigate this option, it adds a great deal to the user-friendliness of the forum.