The drivetrain as a whole, the exterior is awesome on the coupe. But the best to me is the interior overall. The instrumentation won me over, at first I was leery, but after the test drive I was sold. Compare to the civic to anything in its class and it is by far superior. For the $$$ I could have had leather in the mazda3, but overall feel of the mazda3 was not great.
I really like the instrument panel as well, its so integrated now with the outside tempreture, oil life, and digital speedo. Also the side curtain airbags are a really nice safty feature you only see on really nice cars. Also I like how the seat lowers and raises to accomadate short and tall people. I'm 6'5 and have plenty of head room, that I didn't have in other similar cars.