There's no need to compare what WAS with what IS. The law changed, and what is legally available now is the KLX450R, CRF250X, CRF450X, WR250F, WR450F, DR-Z400, and various KTM 4-strokes.
What is available new at the dealer AND legal to ride off-road (not a "competition only" model) is pretty limited on choices today.
Keep the law in mind, and any talk of buying a 2-stroke or 4-stroke MX bike and modding it is thrown out the window. Taking off the airbox lid or changing the exhaust pipe on a KDX converts it to "competition use" only. If you want to compare stock to stock, then that is fine, but when you make such changes, you throw it into the same category as a "competition only" modded two-stroke or 4-stroke MX bike.
For the price, the KDX was great. Some other bikes sold at the time were better (KTM 250EXC for example), but for a bike designed to meet its price point, it should be. How awesome would a $4,200 MSRP KTM200EXC have been!? KTM didn't have to meet such a low price point, and the bike (and price tag) shows/showed it.
Don't just compare apples to apples, but Granny Smith to Granny Smith and Washington to Washington, etc.