My pin-bridge guitars always get the pin-holes slotted (by me) and the thermo-crappy-plastic slotted pins replaced with non-slotted ebony or bone pins - not for tonal reasons (I'm not a believer in pins affecting tone much - strings, saddles, and picks have considerably more effect) but in order to preserve the bridge-plate and prevent the kind of damage GUEST above referred to.

Any guitar with that cheap 'n' nasty-looking matt finish on the neck and headstock gets it buffed out to gloss.

I also replace heavy enclosed tuners with open-back Waverlys, as a weight-saver and because I love the 'vintage' vibe of open-backs. The exceptions are my Lowdens, which I've put Gotoh 510 tuners on - those are superb tuners and they're going nowhere!

I've replaced pick-guards on my Martins, from the horrible pixelated-pepperoni-looking things they came with, to 'Delmar' or 'Tortis' guards.

Every guitar I ever buy gets a neck-heel strap-button added as soon as it arrives in my house.

And I've put K&K under-bridgplate transducers in all my guitars, with a standard 1/4" end-block jack.

So I guess I'm a 'tweaker'! ;-)