Last year, in January, the OneBox was enriched with a static map, information about reviews, directions. To make room for 10 results instead of 3, Google removed the addresses, the links to directions and the ratings.
It's very interesting to see that the other general search engines (Yahoo, Live Search, Ask.com) link to pages from their local search engines, while Google links to the homepage of each business. Another difference is that Google's box has a variable position, depending on the relevance to user's query.
Search Engine Land asked Google why it decided to increase the number of results. The reason is that "many people didn't realize there was additional local content available beyond the three listings, despite the more results... prompt. Accordingly, Google said that with the 10 links it is hoping to signal people that there is much more local content a click away. The ranking of those ten results is based on a range of factors, including the query, proximity, availability of ratings/reviews and their quality and several other variables."
