I played Chaos Theory, i can't think of something "major" other than graphical enhancements and a few new "tools" over previous versions. (maybe a few new moves too, but nothing that'll change how you play the game).
Game plays exactly the same, i'd say if you're bored with Splinter Cell in general then you'll probably remain bored with Chaos Theory. It looks awesome yes, but i'm not one of those that fall for graphics over gameplay.
It still remains one of the best sneakers out there [other than the Thief Series] though. I'm not even aware of any other game, so i'll probably always take a shot at it until something else comes out.
So i'd probably say Chaos Theory is to Pandora as Pandora is to Splinter Cell I in terms of improvements. ALot of small things, maybe cooler AI here and there, some more interesting settings [climbing up a lighthouse with that huge light ... etc], a few extra "bonus" stuff to do during missions [mostly collecting stuff] and a few more ways to incapacitate enemies [hanging upside down from the roof, breaking a guy's neck]
.....but yea i would the say the "novelty" wore off , still a great game to play though.
There is the multiplayer part too though to squeeze out your money's worth.