Is it it because they're a rival in the NL East? Is it because the Mets take them for granted? Is it because they have such a hardcore fan base? Ok, that last one was ridiculous.
In my humble opinion, it's none of those things. It's a mental issue. The Mets have proven they can easily handle bad teams like what they did against the O's and Indians and Cubs earlier this year. For some reason the Mets have a mental block when it comes to playing the Nats and Marlins...on the road. At Citi Field the results have been different, but playing these rivals on the road makes the Mets a different team.
The Mets have to overcome this in 2010 to make a run in the NL East. The Nats and Fish were the main hurdles that uttered the Mets demise the last two weeks of the season in both '07 and '08. It's not an issue of talent because the Mets roster is superior to both of these squads, but the Mets need to fortify their minds and prove to themselves they can handle these teams on the road.