National Harbor is mainly a convention complex focused on the Gaylord, with a growing range of chain hotels and restaurants. There are ferries from the complex to Mount Vernon and Old Town Alexandria in VA and to Georgetown in DC, as well as sightseeing and dinner cruises on the Potomac. Last time I was there, there were signs for a Cirque du Soleil tour stop, so I guess there's some kind of theater or arena. If you want to have a big meeting and you don't care about being near gambling, the beach, or theme parks, it's probably worth a look and will only get more attractive as more developers buy in.
But National Harbor won't be a family vacation destination in and of itself; there's just not enough to do within the immediate area. Moreover, it's not a great base of operations for a Washington, DC, vacation, since there are plenty of other established lodging/retail enclaves in the DC area that, unlike National Harbor, are on the Metrorail system.
So the only reason I can see for Disney to invest $11M in land at the site is to grab a larger share of the national convention business. And, as the price shows, they're making this move at relatively low risk. Breaking into other top convention towns where Disney is not already located (Vegas, NYC, Chicago) would require much larger investment plus all the headaches of operating in a major city center. National Harbor offers a non-urban setting with brand new infrastructure, along with most of the prestige (and perennial convention business) of Washington DC. It's a smart move.
