What will I need to consider when modernizing a pre-1990s steel frame? I'm in the process of upgrading a well loved 1970s steel frame from a downtube shifter system to a modern 'brifter' system.  

Through this process I've had to look at many disparate sources to get information from all over the internet.  Generally, LBS have been rather sketchy about this - they'd rather sell me a new bike.  I think it would be ideal if we could have one question under which we could address as many of the general problems &amp; solutions that the process throws up, and maybe have a collection of resources that someone who wants to follow this process would be able to refer to for more specific problems.  I'm going to provide an answer myself, once it's established that the question is valid! My question is:

What potential issues will I need to consider when modernizing a pre-1990s steel frame?

Please note, I am getting brifters. Answers which say 'you'd be better off staying with downtube shifters' are not helpful in the slightest.
I think it depends on your frame, I have a 1973 Peugeot UO-8 which I replaced the 27inch wheels with 700 28CC and even replaced the handle bars with modern alu bars from MEC. The seat is now a Brooks and because the seat post tubes on french bikes are smaller I had to use a BMX chromoly post. My bottom bracket is a new square one from Velo-Orange instead of the cotter cranks of old. I decided to go with a flip flop hub but it would easily accomodate a 10 speed cassette. I installed modern TekTro brakes to replace the original Maffac whcih where truly appalling. And altthough I am sure there is a way to convert the down tub shifters,  you would be better off sticking with newer shifter at the same location. 

I upgraded and converted my steel frame becuase it is so rare that I find a bike that fits nicely and I really wanted a steel frame for commuting. You might just be going to the wrong LBS. I went to two awesome shops in MTL that where quite please to help out with this project bike. I do have to warn you that older parts for French bikes are not cheap. In total it cost me about $300 to convert the bike and I propbably spend another $200-300 a year on it in parts, tires, chains and BB (fixie is murder on the BB).

Good Luck and have fun with it.