Lots of great answers and information being posted here - thanks guys. And well done for noticing my accidental transposition of STX and Deore
I'm a big fan of modern running gear on retro frames as long as the finished bike looks right.
I tend to use SRAM rather than Shimano, mainly because I don't like the look of the modern Shimano cranks.
I use modern gear for a number of reasons, mostly practical, as there are times when finding exactly the right XTR rear mech can be a time consuming chore, and you cannot be sure of the quality / reliability of a 20 year old mech, even after paying serious money for one.
I blew up a M901 XTR rear mech a while ago. The blow up was caused by an alloy jockey wheel axle and was terminal. Had I been using an X9 mech then there would now still be one more M901 in the world, and I could have got a replacement mech on line that day.
There are builds where age related parts are best, but most frames look good and ride well with modern running gear.
Ignoring the new disc brakes of course, how much better would you say modern groupsets are compared to mid-late 90s kit?
They are no different in function and not really any better in performance.
You can only re-invent a derailleur so many times. As mentioned before, all the Shimano rear mechs from around 1986 to 2009 will quite happily interchange with each other. A '9spd' mech will work just as well in a 7spd system as 7spd mech does with nine. Anything currently marketed as 8 or 9spd compatible can also be used.
Shifters - 'felt' right from about 1994 onwards, so almost 20 years of the same thing over and over again. Early STi had a bit of a bad reputation because of the grease used. This hardened into a cheese like substance and buggered the ratchet mechanisms. A quick soak and they work again for another 20 years...
The Cassette hasnt really changed since the 7spd Shimano freewheel came about. Its still ramped to aid shifting and again, you can lob a brand new 8spd cassette on a 17 year old freehub and off you go.
Chains are no better, you get what you pay for - some snap sooner than others, some seem to last 5 mins. Exactly the same as 20 years ago. SACHS chains seemed to last ages over Shimano's offerings.
Chainsets - still no better than before, still get chain suck and odd wear. BB's has pages of arguments over what method is better as some external BBs seem to last a couple of rides where another lasts a lot longer. Old sealed Shimano BB's either lasted a week or ten years. Old cup and cone BBS could be sealed for decades and never wear out whereas some cheap stuff would loosen after 5 minutes (literally!)
Hubs are not really much different - bigger bearing area with larger axles but how often do you actually wear something like that out?
Headsets, again, they went bigger with reports of premature wear - same as god knows how long ago. Some headsets can be fit and forget, others....!
The whole thing is just trying to keep itself 'fresh' for a dwindling market. If you are reasonably skilled with an allen key and a cassette removal tool, you'll be able to keep a bike going for as long as cassettes and chains are available.
Its up to you to decide what you want - a lot of 'old' stuff can still be used and serviced as if it was released yesterday - I was running 'old' gear commuting 30 miles a day through the winter. The cassette and chain were kept clean and are still usable whereas the neighbours kids bike has simply fallen apart after a year because it wasnt looked after.
Once you go into the dizzy realm of 10 spd and 1:1 shifting, it all gets a bit messy. I am looking at a 10spd SLX cassette mated to a triple chainset and it looks mad...
But, hey, thats what the marketing folk think we all want so thats what the must have should be.