What are the differences between the Fluxx games?
Ecofluxx has a much bigger emphasis on "X eats Y" goals; where if you have X and anyone has Y, you win.  It theoretically makes it a little easier to win the game.  It also has a card named Extinction (I think?), to permanently remove any one keeper from the game.

Monty Python Fluxx has a lot of cards that say "Do this on your turn to earn extra rewards", e.g. Speak in a funny accent to pick up another card.  It makes the game more...  Interactive.  It also has a number of the 'knight of the round table' keepers - they are often interchangable for goals.  In the same way as "Food", sometimes it is one specific knight, but often it is "X plus any knight".

Stoner Fluxx has a number of cards that are meaningless unless you have marijuana available (e.g. "Everybody Toke!"); I suppose you could either remove them from the deck or have a house rule you can play them as cards when you really don't want to do anything else.

The upcoming Pirate Fluxx sounds like it will be awesome and different, with an emphasis on an incredibly strong keeper "The Captain's Hat", that provides the player with a number of bonuses, along with everyone else receiving +1 Draw / +1 Play.