While this article is quite interesting, it is very limited in its approach. What I mean is this - in essence it says, because we notice that the carousel results have a lot of reviews, we hypothesize that lots of reviews gets you in the carousel. Then we set out to test that theory and found out it was confirmed.
While that is useful info, it paints much too small a picture. This year's annual report on Local Search Ranking Factors goes into far more detail (http://moz.com/local-search-ranking-factors). It is a long article, but if you just search for Carousel you'll find the relevant bits (BTW, while this is the initial report, with the opinions of their international expert panel, the full data is normally released later, with full statistical correlation).
In the chart, you'll see that David Mihm of Moz concludes that there are several major factors contributing to presence in the carousel, and reviews is indeed one of them. Much bigger signals include (1) "place page signals" (from your G+ pages), (2) external local signals (local listings), (3) on page signals (your website content), and (4) link signals (links to your site from local sources). Of lesser weight are social, behavioral, and personalization signals.
What does it all mean? These five areas (the four above plus reviews) need to be areas of constant attention for small businesses. At a minimum, you need to (1) keep your G+ pages up-to-date and accurate, (2) make sure you are listed and linked from all high-quality local sites that are appropriate for your business, (3) your website contains quality content that is relevant to your searchers, and (4) get as many reviews as you can around the web, but most importantly on Google.
While these steps won't guarantee inclusion in the carousel, they will result in maximum placement in the SERPs and in Local Search (which appears on mobile devices, specifically, as well as on Maps), and may also get you in the carousel, if your area has one (the carousel doesn't appear in many locations - there is some speculation about when it will and won't appear).
As you search for "carousel" in the Moz article, check out the comments. While the experts do not all see the same value and the same future for the carousel, most seem to feel that it presents a great opportunity for a small business, with excellent local search placement, to appear "up there with the big guys."
Scott.