I think, if you're going to be specific to Monkey Island or Day of the Tentacle (or Nancy Drew even, maybe) is that they try to tell a genuine adventure story featuring genuine characters who actually fit, and want to be there. I mean, with Guybrush, Bernard, Indy, Luke Skywalker, and Nancy Drew and the Hardy Boys, they all intentionally put themselves in those situations because they
want to explore, do good, find the truth, go on an adventure.
These characters are, in essence, awesome. They make mistakes (Luke Skywalker, Nancy Drew are newbies, or at least underdogs, Guybrush is a bumbler, even Indy occasionally screws up and releases giant balls and spikes that nearly kill him, Captain Jack Sparrow is himself), but are guided by decently pure morals (generally) and a desire to be awesome in all the ways an awesome person should.
They're not full of
BK Kids Club characters wearing retarded mirrored sunglasses who spend the entire game acting too cool to be there. You know, more imagination driven than badass driven...
Again, maybe not "The Reason," but I think it's another big part of it. Joonas' mentioning Star Wars and Indy reminded me of it.
