Atlas was really all Microsoft wanted since Google had bought DoubleClick and Atlas was at the time one of the few other large ad serving platforms. The ad serving business basically consisted of a way to track campaigns, impressions, clicks, conversions for large media buys for Fortune 500 companies. There was also synergy with the Avenue A | Razorfish business since clients like Nike, Best Buy, Hawaiian Airlines, and MSN would use Atlas to track their large campaigns across the web. The expectation from Microsoft at the time was that they could build a large display network across the internet using data from users that visited MSN or Bing just like Google does with it's search.
AA|RF was a bad fit because Microsoft was not in the advertising agency business so they quickly dumped that to Publicis. I worked at AA|RF in client services for a few years and were growing quickly at the time. We were doing things like multi-channel attribution and tackling view-based conversions years before they became a mainstream topic in the display ad world. A lot of those folks have gone on to lead at other places like Bob Lord, President of AOL, who previously led AA|RF!
DrivePM was a retargeting business that leveraged custom cookie pools that could be created based on both user's ad viewing/clicking behavior and tracked behavior on a client's website. So for example, you could say I wanted to target only people who viewed my ad in the last 2 days and visited my checkout page but did not complete their purchase. I think with investment, this business could have grown to take advantage of the surge in RTB and programmatic buying. One of the most interesting programs they had was a "recyling" product that basically took wasted impressions and resold them to other advertisers to reduce media costs. This involved doing a study to see what the optimal number of impressions was per user and the conversion rate dropoff curve per additional impression. Once you had that number, you would set a rule that said if this user has already seen the maximum number of ads from one advertiser in a given time frame, don't show the ad and instead show an ad from the recycler network.
Hope that helps!