Is there a business opportunity for a local all-in-one site combining the features of the most popular 'horizontal' sites, e.g. Twitter / eBay / Craigslist / Yahoo Answers / Gowalla ... ?

I'm skeptical. The reason is that what you're describing is really a feature over a set of quite-different products, as opposed to a coherent product of its own. There are two things that make that hard:    1.) There are tremendous scale economies in product development -- build a Twitter (or craigslist or eBay or Quora) for one location, and your marginal cost of building one for another same-language location is almost or exactly 0. So your individual cost of building a broad-functionality comparable product would be close to the sum of the development across all those sites (Yahoo + Twitter + ...). Note that this isn't true for business development or sales; companies like Yelp definitely have non-trivial location-specific marginal costs. I'm just talking product.    2.) The human mind is built to quickly categorize/pigeon-hole the objects in its environment, including web products/apps. Facebook, Yahoo Answers, Twitter all have/had relatively clear positioning/usage; the less clear a product's use case and thus quick categorizability, the less it gets used (see Google Wave/Buzz). My sense is that when you have a reason to use a web application, you're thinking more in terms of the functionality (ie, looking for a cheap car, or a great restaurant) than a location (which would simply be a filter along with other domain-specific information like car make/model, restaurant cuisine and price range, etc). I also think you're fighting a macro trend in the increasing spatial scope of human attention/reach. The locality of a product that you wanted to buy, or information you wanted to consume, was a lot higher 20 years ago than it is today. The existence of eBay/craigslist/Amazon, along with decreasing shipping costs and increasing trust/logistical support, makes the effective purchasing universe a lot bigger as well as cheaper -- a more liquid marketplace. The same is true for information -- the absolute funniest story of the year in your local newspaper simply can't compete with what's out on The Onion or http://FMyLife.com every week. Likewise, the rise of the national cable news networks has come directly at the expense of local affiliate news stations. As for the sectors (local services, dating) where location will continue to be a factor, the domain-specific sites will always be a step ahead of any broad, unfocused effort. (See Match/eHarmony/PlentyOfFish vs. Yahoo! Personals, or Yelp/CitySearch vs. any of the city-specific review sites).