First hour or so I've noticed the following:

-Threaded sms rocks
-Start up time is easily cut in half (makes me in think something might have been wrong before)
-WAY less lag swtiching between tabs/programs
-SPB programs installed and worked fine, though have not tried MS themes yet (I use Mobile Shell, Weather, Pocket Plus, and Phone Suite)
-and for those that use Mobile Shell there is no longer a lag when the device is turned on - i.e. - it goes straight to the NOW screen instead of momentarily going to the today screen first... that's quite nice!
-I've yet to have to go back and correct messages due to missed letters
-No problems handling 2 bluetooth headsets and switching between them is smooth ( did have to enter passcodes for one headset however)
-PIE(+) (i mean default PIE with Pocket Plus installed...) is now a fine default browser for me until I get into Skyfire beta and/or Opera 9.5 becomes stable (on a side note I've used the last few opera betas and I must say... quite impressive... I'm not sure if it's impressive enough to get me to pay for it however...)
-Auto-detected my exchange settings just fine, which is nice because I'm lazy
-I sort of like the "getting started" thing, though I doubt many people in this community will find that useful)

Also all the SPB software was fresh install. The only other two programs I have installed atm are iContact .75 and Windows Live Search (which is even better now that the device is faster... or at least feels faster).

I think that's about it, but just though I'd share my experiences with 6.1 in case any other poor soul was like me - super hesitant.