Interesting! How you finding the Nikon? Did you have many Canon lenses? Missing them? How are you finding the high ISO performance compared to the 450D? My bug is crappy 800ISO performance. Be interesting to hear some of your thoughts on the camera, coming from a 450D.
Sorry for the delay.
I love the Nikon, the performance is fantastic compared to the 450d in all respects. I don't miss anything about that camera.
Apart from the actual jump in output from the camera, one of the biggest things that I never anticipated is how fast access is to all the controls without menu's etc via the controls, especially without taking your eye away from the camera. The controls all become second nature, so you can react fast, and the camera just delivers. Pretty much anything you want to change can be changed with a press of a button and moving one of the control wheels. The controls are well thought out too, so to me after a bit of practice, it just "fits".
I was asked to take some photos on a friends 550d a few weeks ago, and it was like a step back in time. Managed to do what I wanted, but made me appreciate the speed of the d7000 even more.
I had the 450d as part of the twin lens kit, so had the 18-55 and the 55-250 kit lenses. To be honest, for what they were I was happy with the lenses, can't say they were wonderful, but they were cheap and did what I expected. I did find I tended to be constantly changing lenses and as you mentioned, both struggled in low light compared with the 450d ISO performance. With the kit lenses I struggled in low light at ISO 800, I can now shoot comfortably up to ISO 6400 in similar conditions. I was also blown away by the detail in post production, the dynamic range is considerably extended over anything the Canon captured. Also, you will notice a big jump in focus accuracy and speed. You don't have to think so much about getting the focus right, as the autofocus is very good at snapping onto whatever you are aiming at. A good example is sports shots, when the canon struggled with fast moving objects, especially towards it. The Nikon in comparison does what you want.
In fact, if there is any criticism I can make of the d7000, is that I found it quite unforgiving. By this I mean, if you miss a shot, it's not correctly focused or there is some other issue, it is down to the user, not the camera. There are no excuses with this body I'm afraid!
For the d7000, I currently have the Tokina 11-16 f2.8 (A+ love it), Sigma 50 2.8 Macro(B, Not bad, quite versatile), Nikon 70-300 VR(A-, like it but could be a bit sharper at 300mm and notice the slower aperture compared to my other f2.8 lenses). I just picked up a Sigma 24-70 F2.8 to try out, but will reserve judgement on that until I've properly tested it this weekend.
As I've already mentioned, and as you've probably guessed, I don't regret the swap for an instant. Wish I had done it sooner!