Also a much more focused user base. From an advertising point of view, do you think it's easier to sell 10,000 uniques on a general web technology site or 5,000 uniques on a site that has a very specific audience like web designers?
With a much smaller subset of potential users I think you'll also find a much more "focused" group of advertisers, i.e. a smaller group of interested advertisers.
I can see them sharing the same database for both sites but displaying the CSS/HTML questions both on doctype and SO, but doctype restricted to CSS/HTML only. This will allow them to get content through multiple channels but allow them to sell more targeted ads on doctype.
Since SO content is licensed under creative commons, there's no reason they can't feed in all questions into SO... I suppose it might be tricky to merge votes of the (much larger) SO audience with votes of their audience, and has interesting implications to SO's business, but it's certainly doable.
1) Much smaller user base
2) The Litmus ad on the right side of every page
EDIT: The rest of me sees healthy competition and an interesting take on freemium marketing for their money-making app. :)