My passenger door rattles severely.  Afterward I found it's a
common complaint.  I wish the dealer would have saved my time
and fixed it before I drove it off the lot.  There are other
rattles, but overall it's pretty quiet for its class.

I'd pick the MZ6 seats over the MZ3.  (Maybe even the MZ6
over the MZ3 depending on options.  They're not priced
dramatically different)

The dash.  Of the RX8, MZ6, and MZ3 integrated dashes, this one
is the silliest.  It's the love child of Battlestar Gallactica
and Knight Rider.  I joke with my wife and make swooshing sounds
when I change the radio station (dash lights up left to right).
She says, "Michael, don't touch me there."  Minor complaint.
My wife is more fun to look at than the dash anyway.

The radio is nearly inseperable from the dash without much
gnashing of teeth and weeping.  This would be okay if Mazda
offered more radio options.  Even an aux input would make my
day.  Satellite would have been nice.  

There are some other oddities in the dash.  The LCD's right hand
display doesn't work in the North American release.  (I think).
It has the ambient temp and environment display.  It's left
mysteriously unpowered.  It would otherwise show lcd fan blades,
numerical temp displays, and other stuff there.  It's just
a bitter reminder of what I *could* have had if I lived in
Japan.

Most of the complaints are modification limits.  Who buys a
economy car just to throw a few thousand dollars more into it?
So I question my own complaints.  The seats still feel like
an old sofa tho.  And yes, zoom zoom applies.  The car is
a lot of fun to drive.