Velouria asked a very interesting question: How and why do I make the choices I make when I build a bike?
In brief, I like both form and function. I don't strive for a "retro" look, as I've found that trying to re-create the past--in any area of my life, not just cycling--usually doesn't work very well. But it just happens that many, if not the majority, of the bikes and parts that appeal to me aesthetically are from, or inspired by, the past.
And there are some "retro" parts that, for me, actually work better than their modern counterparts. First among them are steel frames. All of my bikes are built on them. I've had aluminum bikes and have never liked the ride, much less the looks, of them. I've ridden, but never owned, carbon fiber bikes and have ridden carbon fiber forks on my own steel or aluminum bikes (though I don't ride any now). They are light and, yes, I would ride carbon bikes if I were racing and someone were going to buy me a new one every year. Which brings me to one of the reasons why I won't buy one: The hard-training, high-mileage riders who ride them seem to replace them every year or two. As a writer and college instructor, I can't afford that!
Plus, I don't care much for the way carbon fiber bikes look.
Anyway...When it comes to components, probably the most "retro" things I like are downtube shifters, handbuilt wheels (as opposed to factory-built wheels) and pedals with toe clips and straps. For me, they seem to be the most practical options.
Downtube shifters are much less expensive than STI or Ergo or any other "all-in-one" brake/gear levers. So, if you take a tumble, you're unlikely to even scratch your downtube levers, and if you trash a normal brake lever, you can replace it for $20 as opposed to $200.
Plus, I was riding downtube shifters long before the "all in one" levers came out. And I ride friction (non-indexed) shifter. Back in the day, that was the only option. When you use it, you don't have to worry about parts compatibility as much as you do if you use indexed shifters. My favorite of the old downtube shifters were the Simplex retrofrictions, which were also, to my eye, the prettiest shifters ever made.
I don't use them now because they seem not to work with more than seven speeds in the rear, and even seven is not an optimal setup for them. On Arielle, I ride Dia Compe "Silver" shifters, which are more or less replicas of the old Sun Tour Micro ratchet shifters. And on my Miss Mercian, I have an old Sun Tour bar-end shifter.
I have owned and ridden the high-end pre-built wheels (e.g., Mavic Ksyriums and Heliums) and while they're light and, for the most part, of high quality, they take all sorts of non-standard parts like straight-pull spokes. Replacements are therefore expensive, require special tools and sometimes can be difficult to find, in part because those parts are usually proprietary. And some wheels, like the type that were made by Spinergy, cannot be trued or otherwise worked on. You wreck one of those, you toss it out.
If you have a wheel built from high-quality components by someone who knows what he or she is doing (or if you do it yourself), you are likely to have a wheel that lasts longer and can be worked on as needed. Plus, parts like spokes are readily available and much less expensive. On my Mercians, I have Phil Wood hubs, Mavic Open Pro Rims and DT spokes. And my LeTour has wheels with Formula/Origin 8 hubs (a "flip-flop" freewheel/fixed gear in the rear) , Sun CR-18 rims and DT spokes.
And, after riding clipless for about twenty years, I went back to clips a few years ago. I simply like the option of riding whichever shoes I'm wearing. Also, I tend to wreck pedals, and clippable pedals are much less expensive than clipped ones. I also tended to wear out cleats pretty quickly, which isn't an issue with traditional pedals.
At the same time, I ride modern components when they are clearly better than the older alternatives. Three of the best examples I can think of are derailleurs, casettes and brakes. Even the least expensive derailluers available today shift more accurately and smoothly than even the best of the older derailleurs, and moderately-priced dual-pivot brakes with Mathauser Kool-Stop salmon-colored brake pads are more powerful and provide better modulation than any older brake, and are about as powerful as cantilevers.
As for cassettes: If you are riding seven or more gears in the rear, they are much better than spin-on freewheels. They provide more support for the cogs, which means less stress on the hub axle. For the couple of years that eight-speed spin-on freewheels were made, bent and broken rear axles became more common.
That said, I use eight speeds. It provides a wide selection of gears, and the chains for them tend to last longer. The more gears you have in the rear, the thinner your chain needs to be.
Of course, if you ride a fixed or single speed, you don't have to think about these issues.
Now, I like the look of many older "quill" stems. However, having taken some long tours, I came to value the practicality of threadless headsets, especially after having a threaded headset come loose in the middle of the Massif Central, many kilometers from the nearest bike or auto repair shop. To adjust a threaded headset, you need one or two large wrenches, which are not practical to carry. On the other hand, you need only a five or six millimeter allen key to adjust a threadless headset. And there are simply more threadless heasets and stems available.
Now, one area where I let form rule over function is in bike bags. I much prefer the looks of canvas bags to their nylon or cordura counterparts, even though canvas bags are usually heavier. But they also have another benefit in that they tend to last longer and are more adaptible. Too many modern bags require special proprietary hardware and accessories to mount them.
As for aesthetics: For the most part, I prefer lugged frames, although filet brazing is often quite nice, and I've seen some pretty artful TIG welds. I also prefer silver components. Polished silver is nice, but anodizing is all right, too. Either way, silver looks classier than black or neon colors in most components. I'm not dogmatic about that, though, as you may have noticed from looking at my bikes: I have black rims (with machined sidewalls) and black chainrings with silver cut-outs. But in most other parts, I prefer silver.
So...I know that almost anyone who reads this will dispute at least one thing I've said. That is your right. But just remember that your riding experiences probably differ from mine. I used to race, but I haven't in years. And I've literally lived on my bike, and I live with them, so I tend to choose accordingly. Finally, you may simply have tastes that are different from mine. Chacun a son gout.