The most significant ones to me are:
TTS
1GHz Snapdragon processor
Android 2.1
5MP camera

Actually, there will be a CDMA version for Verizon, so it's possible it could also work on Sprint. T-Mobile and AT&T use GSM sim cards.

Also, Google says they will be adding many other phone models and networks to their online store.

I noticed in the Q&A that they will be working on a world phone Android model in the near future (both GSM/CDMA). This will be a great feature for people who travel internationally, or to be able to switch networks at home.

For this current model, I think the voice feature will be the most significant: every text field is voice enabled, making the touchscreen-only keyboard a bit less painful.