I have had the touch diamond, touch pro, touch pro2 and now the HD2. The simplest way to compare the TP2 and the HD2 is speed and responsiveness for everything you do is just better with the HD2. With the TP2 there were many times where it would lag when using the Sense interface (I often used the stock 6.5 interface instead because it was snappier). The roms posted on this forum made the TP2 better, but not as snappy as the stock rom on the HD2. Just scrolling across the TF interface, scrolling through contacts, or scrolling down the program list demonstrates a significant "smoothness" difference.
For the GPS, I have always gone back and forth between Bing and the Sprint GPS. So, losing the Sprint GPS doesn't really bother me. I really can't compare Telenav to the Sprint GPS program yet. Maybe others can?
http://m.bing.com/download is by a long shot my favorite app now. It says the HD2 isn't supported, but you can install it anyway and it works wonderfully.
Everyone says that you get used to the onscreen keyboard but there is no alternate universe where the onscreen keyboard is as nice as the TP2. The TP2 keyboard is great. . .of course, the keyboard also makes the phone fatter and gives it the feeling of a brick in your pocket. Never bothered me, but I can see pro/con for each.
$ is another factor in favor of the HD2. Right now, with TMob, you can even get a buy 1 get 1 free. . .with a new contract, you can pick up 2 HD2's for about $200.