I hope I made some positive first impressions myself.
The first raid, which was a full toc25 clear, heroic Beasts, and attempts on heroic Lord Jaraxxus, went reasonably well for myself last night. I did make some uncharacteristic mistakes, like dying to an Icehowl charge because I was stuck behind an elemental and couldn't see him, and a poor Twin Valkyr performance due to aggro issues. Still, I was pleased with my overall work. I only died early a few times - and that's a few times more than normal - but I also did top the meters overall and managed to do the few specific tasks I was given pretty well (occupying the rogue during faction champs).
What about my thoughts on the new guild?
They're not as 'nice' as my old guild. For some, this would be very important. On the other hand, their 'not niceness' was in a raid, and mostly focused on calling people out for mistakes. They were very efficient at recovering from wipes, and overall displayed a more 'hard core' attitude of seriousness about downing bosses. Overall, the healers top to bottom healed better, and the dps was much deeper - while I did top the meters, the 17th dps actually did pretty decent dps as well.
We had some problems getting heroic Beasts down, and Lord Jaraxxus didn't go very well, but all in all for a night's raid, I was pleased to see the potential for progression. We should get LJ down tonight on heroic and spend some time working on heroic Faction Champs.
I'm hopeful - and optimistic - that we're set up well for progression in Icecrown.