I would highly second Hydroguy's opinion about "crowded" seasons. I personally love going during summer as well. The park hours are super long every day (DL:8AM-12AM, DCA:10AM-10PM) and the weather is much more bearable than WDW during the summer, and believe me, I've visited DLR in June, July and August and WDW in June and July.
Again, an added bonus this summer will be the new Little Mermaid ride, Star Tours II, a whole new revamped part of Paradise Pier in DCA, and a new parade for DL! Although it may be more crowded this summer because of these 4 new offerings all opening around the same time, this is probably one of the only times in DLR history that so much new stuff will be introduced all at once, and I would not miss it.