We found the 10 man to have high enough gear requirements to make it not faceroll easy. We started with a relatively weak group and had to sub in for some better players before we were able to clear it. Ulduar 25 geared players should have no trouble at all with it. Ulduar 10 geared players will likely be challenged decently. In particular, it seemed the tanking and healing requirements were a bit higher than initially expected.
Each week an additional boss will be unlocked. You'll fight all of the available encounters each week.
Edit: I can confirm that Tranquilizing Shot does indeed remove Icehowl's enrage.
We tried 25-man on Tuesday, with our regular raid group - (3/9 hard modes in Ulduar down) and 2-shot the boss - very easy.
I do want to confirm that the 10-man, however, is not faceroll easy.
We tried a mostly random group(good tanks, good but random DPS(almost all melee - I think part of our problem), 1 resto druid 2 offspec discipline priests(when one wasn't enough)) and couldn't seem to get it down in ~1 hour of attempts. I have no doubt that with any sort of real group we'd have gotten it, but even outgearing it (full Ulduar 25 gear + some hard mode gear) we failed to kill it in 10 man. 9 out of the 10 players in that group had killed the 25 man version.