Bike changes: What kind of difference does ten years make?

I personally think that groupsets are the one thing that most improved over 10 years. In other words, due to technology trickle, the current iteration of Shimano 105 (for example) is more or less as good as the 2004 Dura Ace stuff was.

I agree with group set comment 100%. Bike frames, harder to quantify. If you're getting a tri bike, then the improvements have been large in terms of aerodynamics and range of offerings in terms of fit. It's also safe to say the ability to get fit properly on a tri bike has changed dramatically for the better. Road bikes, not so much. I don't subscribe to the notion that carbon fiber bikes are any better or worse than aluminum per se, and frame shapes haven't made any leaps on roadies in a very long time. If you're riding a bike you love, can ride it comfortably for hours, and find you are powerful enough atop it for your needs, I would say don't change. If you suspect you could benefit from a small improvement from your gear in those terms, have a look around. As always, the best improvement you can make on the bike is to ride more.

Could you get a mid tier carbon bike in 2004 or were they all at the higher price point? Carbon is much more common today.

Absolutely could get a mid-tier carbon bike in 2004. I can think of several Treks from that year off the top of my head, just one brand. 10 years is not that long ago!