Very few replacement parts are available for Shimano's STI integrated brake/shift levers. Campagnolo's levers (they call them Ergo, Shimano calls them STI) are completely rebuildable. Every single part is available for them to the point that you could literally build a complete lever from the available parts (though it would end up being really expensive). The wear parts are relatively inexpensive, and can be ordered from almost any local bike shop as well as a lot of places online. If you aren't exactly a certified watchmaker, you can also send them out to a couple of places to have service work done, for $40-50 in labor. The most common part to break are the "g-springs". They cost less than $5. You might want to replace the "ergo index gear" if it's worn at the same time. They cost less than $45.

Both Shimano and Campagnolo hubs are completely serviceable and rebuildable. Campy's Centaur through Record hubs are almost identical from model to model, except that Record hubs have a titanium pawl carrier on the freehub body and a grease injection port on the hub body to keep the lube fresh more often. Shimano's vary from model to model, with their more expensive hubs getting better bearing treatments, etc.

Most of Campagnolo's current front and rear derailleur parts are available for replacement. Only a very few Shimano derailleur parts seem to be available.

A lot of replacement parts are available for Campagnolo brakes too, from as far back as 1994. I've found a lot of replacement parts for Ultegra 6500, Dura-Ace 7400, and Dura-Ace 7700 brakes as well.

Neither Campagnolo's nor Shimano's recent bottom brackets are serviceable or rebuildable.