(although I will support the nominee in the general, and am not "anti" any of the three in my sig line).
1) His solid executive experience. I think that will really help him in actually getting things done. A specific thing he did both as mayor and governor is the use of data for tracking and managing government service functions as well as environmental statistics for the Chesapeake Bay. (BayStat, CitStat and StateStat)
2) He has the best environmental record and platform of the major candidates. He has items in his plan like "Prioritize modernizing our electric grid, to support localized, renewable energy generation" that show that he REALLY gets certain basic environmental principles. His campaign platform does not look like something ginned up to just "check the box". It seems that he has genuine, adequate knowledge of environmental issues to make good policy decisions.
3) I like his criminal justice reform platform and the way he handled the NRN protest.
An example article that shows both items 1 and 2 : http://grist.org/politics/martin-omalley-long-shot-presidential-hopeful-is-a-real-climate-hawk/
OMalley is leaving office with a mixed, or at least nuanced, record on fracking. Western Maryland, just south of Pennsylvania, has natural gas deposits that are recoverable by fracking, but they have yet to be exploited. OMalley imposed a moratorium on fracking in Maryland in 2011. But hes about to be succeeded by Republican Larry Hogan, an enthusiastic fracking proponent. So after the November election, OMalley announced that he will unveil regulations this month that will allow fracking under limited circumstances, following the best practices of other states and imposing additional, stricter rules to curb air and water contamination and restrict where drilling can take place.
For some environmentalists, OMalleys willingness to allow fracking at all is their one disappointment in his record. I would prefer that OMalley would come out in favor of a ban on fracking in Maryland, says Tidwell. But others say OMalley is making a shrewd move. With rules in place before Hogan comes in, Hogan may find it more politically difficult to repeal them than he would have to simply not write any himself. The fact that we have a governor-elect who wants to move forward on fracking means we want to get some protections in place as soon as possible, Karla Raettig, executive director of the Maryland League of Conservation Voters, told The Washington Post.
I think this approach indicates a grasp of reality and strategic necessity that is essential in a President, particularly when dealing with opposition majorities in Congress, as the next President almost certainly will be for quite some time (due to gerrymandering and Dems laying down in the 2010 state legislative elections.)