This buy is something Yahoo or AOL would do. Google didn't buy a technology or a business model. It bought content, an address book, and a logo. Very un-Google.
If Google's technology isn't good to compete with Yelp and Groupon, what makes it think it will be any better after buying up someone else's operation that can't compete either.
To quote Vic Gundotra, 'Two turkeys do not make an Eagle'
Google (or any of us, for that matter) can spend all day building the greatest, fairest, most accurate restaurant review system known to man - but it will still be of utterly no use unless there is a critical mass of restaurants and customers who are willing to trust it and, more importantly, use it.
Bootstrapping a trusted content source is really hard (hard in the "nobody knows how to do it reliably" way, not just hard in the "it takes a lot of work" way). If you want to break into this market, simply buying said critical mass is much easier (and much less time-consuming). Whatever crazy technology you have can be layered on top.
I know Zagat as that magazine that fast food places brag about getting high ratings from (KFC ran commercials for a while claiming a #1 rating from them).
I don't see this as a great purchase for Google either. They already put business reviews from other websites on their listing pages. I have seen Zagat ratings on various pages. Why couldn't they just pull it in also instead of buying the company?
I was certain zagat was made up by Ellis for his book.
I'd never seen it until I moved to the bay area, where a 'Zagat rated' usually means "Yes, this is a chain, but we pretend really hard that it's not fast food."
Weather search? Google gives you weather info right in the SERP. Stock search? Same thing. News search? Hrm.
Restaurant search? Why hand all of the revenue from these search results to advertisers and only take a slice of it for the adwords (or NONE from organic results)?
If a lot of people search for it and companies are making money, Google is at least discussing whether they should own the content/service at the top of the search results.