This is an interesting development on a few fronts. Gabe Rivera's suite of memetracker sites – anchored by politics-centric Memeorandum and tech-centric Techmeme – does an excellent job of selecting hot stories (based on an algorithm that uses factors such as links and "influence") and then surrounds them with related stories and blog articles in story clusters. These clusters change and evolve and move up and down the page fluidly, so it's easy to see which stories are hot and being talked and buzzed about across the Internet.
Included stories cover a wide range of subject areas and are selected from both mainstream media publications as well as the blogosphere. The new Mashtracker narrows the focus by just tracking social news stories published by Mashable, and the related conversations that spring up around them within the blogosphere. Mashable has a unique opportunity here to be successful, I'll wager, because it already is a trusted source for social news. (No one does a better job of keeping up with the current blizzard of social networking start-ups, for example.)
And the particular focus on blogs is another step forward in terms of the blogosphere's credibility. In essence, this is another way in which the blogosphere is declaring that it is in many ways a better source of Internet news, reviews, and opinions than traditional media.
While Mashable's layout is similar to Techmeme's, the design is a bit more clunky. That said, I do like that company logos are used to anchor the lead stories. I imagine they'll clean things up and streamline as the new site matures.