the regular klx or klx L? the regular obviously has the new styling, 4 speed, different tires i think, and i think thats about it. the L has a longer shock and swingarm, longer travel forks, the new styling, 4 speed (1 down 3 up) transmission, manual clutch, different tires, and something about the cylinder walls have a different type of coating or something and there was a little something else to help add horsepower, they said it makes like an extra 1-2 horsepower or something like that. my L is a lot faster then my 08 i had (stock and stock). i love my L its a lot of fun but i still love the 03-09 klxs too.
also good thing to note. the 08 klx has phillips head screws holding a lot of the motor together. the 09 and newer is actual bolts. very good thing if you plan on tearing into the thing more than once doing mods. get an impact driver if you take it apart. will make your life easy
the exhaust on the L is the same as the regular 02-09. it just has a stupid little cover thing over it. personally, i hate that cover that comes on them because you come around back and it doesnt even wrap around its just like a fancy heat shield.