We have been watching this for a while. In our research we have tried linking to the citation sites, and have had little success. There is no doubt that the principles of places ranking are related to, but not the same as normal organic.
Reviews are very helpful, and next to the normal important areas (profile completeness, address and others) I would put reviews as top priority. Not only on Places, but Yahoo, Insider pages, city search, yelp, Angies, and even customer lobby. I am sure there are more. Recently Places started displaying reviews from the Places accounts owners web site. Seems if you use reviews or testimonial in the URL they may pick that up. This may be an attempt on Google's part, to slow down the number of duplicate reviews. (Those with ears , let them hear). <- not sure what I mean, but I have only had one cup of coffee so far.
Look at the simple fact, that you will rarely find a places account on the 2nd or 3rd page of results that have a significant amount of reviews.
We have stopped using the "usually suspects" for citations, as in the last 5 months they seem to have little value. What we do see working is citations on other "local" sites. Not directories, but related local sites. Much harder work, but Google likes it when we work hard for a citation or link... right?
PS- We are not all that large in SEO, just do local clients , so our research is very limited.