Was anyone expecting an active geology?

There was a little bit of speculation because of the atmosphere that was detected by the Hubble, but largely, no. That idea was mostly ignored because we weren't sure what could CAUSE an active geology on an object as small as Pluto.

And yes atmosphere is being mentioned, and that can be a factor, but we are talking thinner than the Martian atmosphere here.

Yes, Pluto's atmosphere is incredibly thin, but something to remember is that while on something like Mars, the only means of the atmosphere destroying evidence of craters is erosive factors like wind, on Pluto the atmosphere is also in equilibrium with the ices on the surface. Ice will "frost" out of the atmosphere and blanket the surface, covering up a lot of features. The ice is melting more on the parts that face the sun and depositing more on the parts that are not. This difference isn't a lot, but it's enough to add up over time. Pluto is also, as you mentioned in your previous post, orbiting "on its side", which means that this deposited ice is melted off of the summer side and redeposited on the winter side over and over for each orbital period. This type of cycling could easily cover up evidence of craters.

this preliminary data is showing it acting more like a comet than a planet.

This was expected. Compositionally, Pluto IS a comet, just a very large one. The tail was expected when we saw that the atmosphere was so large. We expected its atmosphere to be in the process of being depleted. The tail always points away from the sun, so we couldn't have detected it until we got behind it, this is just confirmation.