Two sites that could really benefit from massive Google Maps mashups are TripAdvisor and MenuPages.
Menupages is a pretty great site for finding a restaurant. You can tell a lot from the restaurant's menu (no more having to deal with vague notions of $$ and $$$) and customer reviews (though it does entail some wading through shills and people who seem to dine with the aim of picking a fight). The one real problem I have is that its neighborhood divisions are far too coarse. Most of the time I have a specific place that I'm already going to be in, for a movie or whatever, and I need something within pretty close proximity, say 3 or 4 blocks. I choose "Upper West Side" and I get everything from 59th to 115th street, from CPW to the River. Hundreds of restaurants. What I desperately need is to be able to sort the list by proximity to my given address. And if they could do that, it would probably be trivial to have a Google Map of everything.
Google does have their own local business finder which can be used for restaurants, and it shows you a pretty good smattering of reviews at a glance, but their system has its own annoying quirk. It also refuses to show you the closest restaurants to the address you provide. Instead, the first page of 10 results will have maybe one or two that are quite close, then the rest will be scattered around up to 10 blocks away. Go to the next page and it's a similar distribution. The last time I used it, it seemed to choose the most expensive restaurants first, but I can't say for sure if it always behaves this way. Perhaps they chose this to avoid maps with too many little pointers cluttered together, but I don't think it's the right compromise.
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Tripadvisor has become as indispensable in my travel plans as MenuPages has for my dinner plans. But I end up in the same routine--read about the place, and if I like it, check it out on Google Maps for proximity to public transportation and places of interest. In this case I sometimes have to massage the address to deal with language and local street naming conventions. I'm sure for both of these sites it would be a large and tedious project to check all their addresses and ensure proper placement on the maps. Existing agreements could pose a problem as well; I see that MenuPages at least has Mapquest links. There may also be limitations to how many locations can be placed on a Google Map. But if it's technically feasible I think the benefit to usability would far outweigh the costs.
PS I'm amazed how many sites still used maps from Mapquest, Microsoft Mappoint, and others even when Google maps had blown them all out of the water--did they all get roped into onerous ten year agreements, signed in blood? I admit the others have caught up a bit now, but they're still not as good.