Here are some of my reactions after about 3500 miles. For reference, my previous cars were a 2002 Bonneville SSEi and at 1997 GTP.

1. The rear view mirror seems large and low, and blocks a large area where traffic might be hiding, epecially in parking lots.
2. I didn't like the automatic dimming mirror, so I don't really miss it now, but the 90 degree turn mechanism on the 04 is silly and awkward. What was wrong with the old "flip up" lever?
3. I never liked the automatic temperature controls so good riddance to them.
4. The Bonneville trunk had the "crush your packages" feature, I'm glad the GP still doesn't. Also the Bonneville would drip water from the trunk lid into the trunk when you opened it in the rain, a less-than-brilliant design. I'm glad to say the GP doesn't do that.
5. That's a reeeealy big control in a conspicuous place to adjust the HUD, which I used once and will never touch again. They have moved the HUD control on every model but this one seems overdone.
6. I like the Cancel feature on the cruise control. I did accidentally turn the wipers on once when trying to set the cruise.
7. I guess TAPshift is ok, but I never drive with it. The interface is better than on a 97 Boxster I had, but shift controls belong on the floor, and there are other cars that put them there.
8. I never used it on the 97, but I still felt cheated when I learned that the 04 doesn't come with a valet key.
9. Speaking of keys, Pontiac mailed me an "emergency key" for the 97. When I traded it for the 02, the salesmen tossed that key out, saying they were nothing but trouble. OK, I didn't get one in the mail for my 02 or 04, and I guess OnStar is a good replacement for the emergency access problem. But if the key is trouble, why did my girlfriend just get one in the mail for her new (OnStar-equipped) Ion?
10. Speaking of mail, Pontiac also mailed me a GTP key ring for the 97 and made me feel like I was part of some cool club. Owning the Bonneville made me feel like I was old, so I didn't expect the same kind of thing. But hey I'm young again now, but no similar treatment.
11. Speaking of key rings, my keys were constantly banging against the dash. I found a Pontiac logo ring on eBay with removable sections (to give the valet! :-) and I now separate the car key so it rides alone. But that's a hassle that I now fiddle with every time I get in or out of the car.
12. Why do the doors lock when I shift to D (and not R) but unlock when I shift to P? The 02 and 97 would lock when shifted out of P, and unlock when shifted to P.

Well, that should be enough for now :-)