A lot depends on where you're going. When I toured heavily, I pretty much just carried the tools to fix and patch flats, lube the bike, and a multi tool and allen wrench set to adjust things on my bike as needed. I didn't carry spare cables, tires, spokes, chains, etc. because I knew that I'd never be too far from a bike shop and I'd rather trust those sorts of repairs to a professional (and would rather not be dragging the weight around for things that I likely wouldn't need). I knew that even if I did do a quick repair by the side of the road, I'd still take the bike to the next LBS and make sure it got done right.

But I was traveling in populated areas, never more than 30 or 40 miles from the next town, so it wasn't likely that anything much would happen that couldn't wait until I got to the next town (and didn't--I've had just a few flats, had to adjust the brakes and derailleurs a few times, and had to tighten a few attachments. The worst damage to my bike - stolen handlebars - I wouldn't have been able to fix on my own under any circumstances!). If I were in less populated areas, I'd start adding to my tool list and bolstering my mechanical knowledge. But I've also discovered, after reading way too many journals on Crazyguyonabike, that most people, when they run into mechanical problems, either put up with the problem until they get to the next LBS, or do a quick field repair to get the bike as far as the next LBS, or they call for assistance. It's a balancing act between wanting to be prepared and not wanting to overpack, and where the balance lies depends on the likelihood that you'll be stuck by the side of the road with no way to get the bike into a shop.