My wife had a SLK 320 and I never liked it; I am very small but feel cramped in it. I don't feel that way with the SLK 350. It no longer looks like a chic car. Handling is much better and it does feel sporty. All the creak and groan are gone with the top up. Top folding and raising are much faster than the SLK 320. The freeway milage is surprising (27.4). Some people may like more storing space but I think the CD changer in the glove box is more convenient. The flip top features of the arm rest is also convenient. The automatic partial rolling down of the windows may help to prevent the leaking of the door weather strips (as in the SLK 320).
The convertible top. The seven speed automatic. The memory seat and steering wheel fearures. The acceleration when passing. Air scarf.
The gear shift is too close and the right side of the seat back is in the elbow way for sport shifting. The automatic setting of passenger side mirror is inconsistent. The cupholder should be moved down (it does appear more solid than the SLK 320's). Seats should have adjustable lumbar support.