Depends on what your application is and how old we are talking about. Are you salvaging the parts to use or to sell. Sometimes it's more worth while to sell certain things whole. I for one personally collect vintage calculators (1960s-70s). Others might be after antique radios or TVs or other such devices. Some people can be tempted to salvage them for valuable internal components like nixie tubes, but sometimes with particularily rare machines, collectors are more interested int he machine than the parts, even if the unit doesn't necessarily work.
Me personally, I save any and all germanium transistors and diodes, as they are very hard to come by these days. Though I save them, they really are only worth using for repairs of old tech. germanium diodes do have a lower voltage drop (around .3 volts, vs silicon's .7 volts) so you might find a good use for them in some applications. I also save hybrid modules, which are sort of like mini circuits on epoxy dipped ceramic plates. Those can be very hard to find, but typically are device and manufacturer specific. Again, mostly only useful for repairs of vintage devices. I also save any tubes I come across.
Are you a hobbyist or someone who does projects? LEDs, switches, etc can be sure fire salvage saves that almost always find re-use. Depending on the types of projects you tend to do, look for parts you tend to use frequently.
You mention Moore's law... Are you talking about PC hardware? I guess as long as parts work, there might be a use for them. Even an older PC might serve as an email machine, or a media box. If it functions, a wipe and a post on something like craigslist or freecycle can give it new life for someone else. As for parts, I am using a pair of power supplies in my new PC build. I'll evenually replace them both with a nice new one, but with a simple jumper wire tying the activation pins together, you can have one motherboard activate two supplies, and use the second supply for things like hard drives, etc. Alternately, just adding a simple switch between the activation line and ground can give you a nice stand alone power supply for projects that supplies 3.3v, 5v, and 12v for projects (I think there is also a low current -5v supply as well). Fans and heatsinks might find reuse... or you might just recycle the heatsinks... It all depends on whether you think you can use them or not.
Modern electronics has fewer and fewer generic discrete components than ever before. In the past, you could repurpose nearly any part of a machine. Now a days, parts are extraordinarily application specific. In general, when LSI intergrated circuits started displacing discrete components, products began to have fewer and fewer user reusable components inside. High density SMT tech makes reuse even more difficult.
I'd actually say, if you are into old electronics, the older stuff is more reusable than the new stuff... by orders of magnitude...
If you are talking about modern devices liek PCs and such... The newer the better, but as long as you can make use of the parts, then just use them. I have an old 9 year old Mac. It's slow as can be, but it still is fine for email and can play video and such. Yeah, it's slow, but it's not without use. Just consider what the best use for something is, and roll with it.