There is also apparently a run of barrel plates made from lower melting point plastic. Both jpc and myself have encountered melty plastic when bolt modding.
But I have 4 IBM Model Ms, a Customizer from Unicomp, and a terminal board I dismembered for parts. I've also given as gifts 3 other Model Ms (2 Lexmark and 1 IBM).
For me, it was more an issue of keys and keyfeel. The ones I gave away all had 1 piece keys, and I do honestly prefer the feel of 2 piece keys. My Customizer was actually my first BS keyboard, and I bought the Model Ms after I saw ripster's "international" mod, so that I could modify them to my "correct" layout, and pay less than the cost of a customized Customizer (which is now nearly $100).
Plus, there's something undeniably awesome about the Model M. The feel that I could use it as a weapon to defend my home. The reaction of my office mates of "Oh! I want one of those!". The nostalgia factor.
And let us not forget the "you are using *that*?" reaction from the IT staff. Plus, properly maintained (bolt mod, swap bad springs, etc.), these things will last longer than we will. Especially since people have done controller swaps, in case the controller wears out. It definitely is the most fragile piece -- see M2 for examples.
In regards to the differences in years, I can say they are noticeably heavier the older mine are. I mean the physical mass of the board, not the key feel.
ch_123 is definitely correct. Wear and tear on the springs is really much more important to key feel than age or model number.
And if you ever decide you want a knew one, take a look at the Unicomps. Worth every penny, if you really want a new keyboard. Plus any idiosyncratic key layout desires can be dealt with if you buy new.