I never got around to installing a beta of watchOS 2, so these are legitimately my "first impressions" of the update.
- The update process took entirely too long, which is probably the reason that the update required the Watch to be plugged in while things were downloading and installing. This wasn't too annoying for me, but I work from home and have easy access to my charging cable.
- New passcode entry is really nice. Buttons seem much easier to hit, and are more responsive.
- Siri has been incredibly unreliable. Even after a restart.
- Capturing screen shots does not work. (edit: No wait, I guess they work, but they just take forever to sync initially)
- Everything first party feels really snappy.
- The app that I was most looking forward to being updated, Strava, did not receive a watchOS 2 update and in fact does not launch on the new OS.
- I plan on using the hell out of the new Nightstand mode.
- I'm starting to really like the capitalization on "watchOS". Phil Schiller was right.
Overall, lots of good improvements, but I'm surprised that people are saying that things feel more reliable with watchOS 2. I look forward to seeing some of these bugs get ironed out and some of my favorite apps getting updated.