The 210 did not have leading edge slats. The tail was too short, so it had some yaw instability with regards to spins. However many of the bad traits were due to faulty landing gear design, bad brakes, and such. It had many problems crashing during landing when the brakes simply failed to stop the plane like they should, or the gear itself just collapsed for no reason.
Messerschmitt was rebuked over these failings. Such a basic principle, but so poorly executed.
In regards to the fixes:
The engines were longer, the nacelles also longer, and this changed the balance of the plane as well as the center of gravity (guessing on that). The leading edges were changed to have slats. The tail was lengthened. The increased horsepower helped the handling (or so I have read) at slow speeds near the stall.
All of these changes were implemented into the 210 production models. It was really the first batch which was "bad" (and even then pilots still wanted it). The only difference that I can see the 410 added which the 210 did not already include is the straight leading edge of the outboard wings (the 210 had a kinked wing, the 410 did not). [edit: and of course the engine nacelles were shorter on the 210A, 210C, 210Ca models]