Hi, mate.
The Zed has been designed to be an old school muscle-car type sports car. It's not a car built just for posing like an Audi TT and Nissan have engineered a certain amount of heft to the controls - steering, gearbox etc. - to appeal to those who would rather drive than cruise. Similarly the ride is firm (but a darn sight smoother than my Mini Cooper S) in the interests of handling. As a result it's not at it's best in town but show it an open road and (hopefully) you will understand what the engineers were getting at.
As for stuff in the boot, I'd recommend a luggage net. Sorted.
Getting in and out of the car I can only suggest you're more careful ( ) - I'm 5' 10" and have no problem. I can't help with the seat comfort and belts although Autocar recently complained about something similiar with their long termer and their somewhat expensive solution was fitting the NISMO sports seats. You do lose the electrics and leather though.
I don't want to appear rude but didn't you test drive the car before buying? What were you expecting and what type of car did you move from?