[StarEye]

Monkey Island 1 - able to explore the governor's mansion more.
Monkey Island 2 - able to explore the governors'* mansion more.
Curse of Monkey Island - Elaine and Guybrush doesn't get married, graphics more in style of Monkey Island 2 with the shorter and not quite as slim Guybrush, and more of an oil-painting look. Less cartoon-style, more "gritty" and "dirty".
Escape from Monkey Island - graphics are WAY too cartoony now, and the story is horrible and completely nonsensical when put against the other games in the series. To be blunt, make it more of a Monkey Island game and less of a "zany cartoon comedy" spin-off that it seemed to be. Too many wrong things with the game that it would take ages to list them. And I even like the game. It's just that it's only Monkey Island about 1/4 of the time.
Tales of Monkey Island - map puzzles, improved "cinematic camera angle", controls, inventory system, story is continuing the zaniness. I mean, a magical giant sponge? Really?

Only games in the series that I have trouble finding flaws with are 1 and 2 really. It's just two perfect examples of adventure gaming for me.

*Elaine's and Phatt's