I definitely have a 2.0 Fluxx then.
Ah, but is it an A, B, or C version 2.0 deck? You can only tell the difference by the tuckbox... versions A & C both have the infamous "I'm Terribly Sorry" typo on the back (omitting the final sentence from Zarf's description, "No one has played a Goal yet") but a C deck also includes a plug for our website, LooneyLabs.com. The B version lacks the URL but does have the missing final sentence.
Not that it matters, but as long as we're talking about trivial differences here...
I think the box design for 2.1 has the Mensa award stuff on it as opposed to the 2.0 box which does not.
That's true. Also, Version 2.1 decks clearly state on the underside "FLUXX Version 2.1" whereas Version 2.0 decks were published by I.C.E. and bear the Iron Crown logo. There are lots of little differences but the gameplay is the same.