I'm 42, so I guess I'm one of those "old-heads" who first started watching TOS during the initial syndicated reruns. I grew up on TOS and it's always been my favorite series, although I've really enjoyed every version of Trek since then (well, not so much Enterprise, but hey...)
It's weird - I'm not at all upset about new actors taking over the roles. I said my fond farewells to the original cast at the end of The Undiscovered Country. I've loved each of them for the past 30-plus years, but there's no denying that they had reached the point where it was time to move on and just appreciate what we had for all those years.
In fact, I'm actually very excited about seeing how the new actors interpret such well-known - and yes, iconic - roles. In particular, I'm really looking forward to seeing how Zachary Quinto steps into Spock's shoes. I've enjoyed his performances on Heroes and he seems to have really jumped into the role of Spock with great enthusiasm. And from what little I've already seen, it appears that Karl Urban might be a really great McCoy. I'm actually excited about the new actors and look forward to their interpretations.
On the other hand, I'm way, way, way more upset about the changes in ship design, set design, costumes, etc. I won't even get into the canon arguments, but the radical and unnecessary changes in the known Star Trek universe bother me a whole lot more than new actors.
It was necessary to change the actors, if there was ever any hope of continuing to tell stories about the original crew, and I'm ok with that. In fact, that's actually exciting!
But seeing those very familiar characters in such radically different surroundings is much more of an issue to me...