My main complaint in Rtw, and I haven't played it extensively, mind, was that it's too easy. You require very little tactics especially on the battlefield. Cavalry were a very conditional force, in RTW you can confidently take a majority cav army into the field and wipe the enemy out, this just didn't happen. That saying I love cavalry in MTW, they just require more skill to use effectively.
The campaign map is very exploitable, there are things in MTW that you can gank the AI with, but with RTW it's almost impossible not to.
Neither are perfect though.