Another Happy TP2 -> N1 customer here. Few points:
Speed - the N1 is VERY quick compared to the TP2 (stock TP2, running WM6.5/Sense 2.1). No lag, everything works instantly on the N1. Phone doesn't slow down after a few days uptime, there's no lag in SMS or MMS.
Keyboard - The major concern moving from TP2 was the lack of hardware keyboard. That was one of the major factors in my decision not to get the HD2. Hardware keyboard IS better for typing longer pieces of text (i.e a decent length email for example) but for the standard phone texts/mms/short mails/web browsing/IM the N1's keyboard is fine. Enabling haptic feedback and using in landscape mode makes it a doddle to use, rarely now have to go back and correct words etc. Yet to try non stock keyboards, these should aid the transition even more.
Look and Feel - The N1 is MUCH more attractive that the TP2, however I sometimes find the N1 a touch on the small side - that being said it's probably more due to being used to the larger TP2, the N1 is certainly easier to used 1 handed than the TP2 tho do sometimes feel like I'm going to drop it!
While HTC's sense was full of eye candy, with a couple of widgets and some time Android can look just as good (and that's before you consider the custom ROM's with sense etc available)
Android vs WiMo - Night and day here (and this is probably the biggest "love" for me at the moment). Everything is at your fingertips. It's so easy to take a pic and whack it on facebook, send it via MMS or email. Check google maps or browse the net. It's all there, and really quickly available. The app store is pretty acomplished too, easy access to a load of apps - something which (last time I checked) the MS stored was lacking in.
Moving from a non smartphone to the TP2 was a pretty big jump. Good net access on the move, easy access to email and other information. The jump from the TP2 to N1 was probably equally as big. All the previous information is available (only MUCH faster) + a whole load of new information, apps and ways to keep in contact with people.
Hope this helps
