Personally, I think the thread preview's are detrimental to SEO.
If you have a lets say you have a one of your forums all about a topic... lets say 'vBulletin' for instance.
By nature, thread titles in the link's URL text will often contain 'vB', 'vBulletin', 'Forum', and other related keywords contributory to the page as a whole.
Now, I know titles (as in title="blah, blah") dont count for that much, but they surely have the effect of diluting the keywords..... not to mention the fact that they add a lot more code to the page and so the code / content ratio's are affected... and the keyword ratio's also suffer.
Just my $2









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