Doesn't place a heavy emphasis on serious commitment of time or thought. Not overly demanding/taxing on players- drop in/drop out gameplay. Friendly to solo players and small groups of players. Etc.
It would be the opposite of say, Vanguard, Everquest or Final Fantasy 11. Classical MMOs in a grand style.
GW2 is the lightest/most casual major MMO I know of short of the ones made for kids or World of Warcraft. I suppose other quasi-MMOs such as Minecraft or Second Life might even have the aforementioned beat. But it depends on how one wants to contextualize those games- they are certainly lighter faire than even GW2/WoW.