Geez, I sure missed a lot of excitement here. A few more observations from yours truly about some of the cars mentioned earlier:
I spent a lot of time lately driving around in a new Mazda3 GT (2.3L) and was really very impressed with how good a car this is for the money! Wonderful, linear power, very quiet for this class of car and it cruises beautifully on the highway at 130 km/h + like any German car I've ever driven. The handling was every bit as good as the last Jetta 1.8T's I'd driven and certainly the shifter / clutch feeling is better (more linear). I would suggest the Mazda3 GT is every bit the (ahem) "sport sedan" that the previous gen Jetta is (haven't driven the new one yet).
When I had my BMW a few years ago, I would get the occasional 320i loaner car when mine would be in for service. Without a sport pkg and its considering its power deficiencies, it was about as exciting to drive as my mom's '97 Accord at the time. Yawn. All BMWs are not necessarily true sport sedans.
The term sport sedan is thrown around more as a marketing term than anything these days. I drove and really liked the Acura TSX, but I would not call it a proper sport sedan. It is fairly nimble, but it's certainly more a value luxury car than a true sporting machine (despite what Car and Driver's 10 Best list says). I would never sneer at someone who chooses one, because they are a reliable, solid and sound machine, but I found it fairly boring to drive. Likewise, my previous Audi A4 with its "sport suspension" could be classified as a sport sedan... though the boring drive was one of the reasons I missed my BMW so much.
A car with a lot of power and slightly inferior handling (ie. the Altima 3.5 SE) is a more exciting vehicle to drive than a lot of these 'sport sedans' since the handling 'benefits' of the so-called sport sedans are so minimal versus a well-equipped family sedan that the differences are barely perceptible. The gobs of power from the 3.5L VQ-series engine, however, is instantly noticable and makes the car feel much racier than its practicality might suggest.
An e36 M3 Sedan is a sport sedan. An Audi S4 is a sport sedan. A Subaru WRX is a sport sedan. Each is a car you could take right out of the box, put it on the track and not be embarassed by lack of power or hideous understeering. Hell even an e46 330i with a sport pkg, fits my mind's eye definition of a sports sedan. Anyone who hasn't driven one, probably wouldn't understand either. Lots of smooth power, good handling, braking, and lots of snarly noise from the I-6.
