I have a friend who never cycle tours because he reckons that loading up a bike with tons of kit and then slowly wobbling off through the countryside is missing the point: riding on the road should be about speed, exciting handling on descents, and so on. He has a point, but is wrong that touring per se is a waste of time-he should go ultralight.
The essence of this style of touring is not to carry anything superfluous, so that everything you need fits into one or maybe two small bags, and weighs a very few kilos. The main thing is: no camping. This means you don't need to carry a tent, stove, pots and pans, sleeping mat, sleeping bag, light, etc. You end up staying in youth hostels (if you're cheap, like us), B&Bs, Gites d'Etape (in France), hotels, motels (in America), and so on. It may or may not be necessary to book in advance, which slightly reduces your flexibility in theory, but at least in my experience much less so in practice. If you camp at organised campsites you are constrained by where they are anyway, and if you don't, there's no chance of a shower. It's often not much more expensive than camping. Eating in pubs and restaurants saves carrying any food more than a couple of chewy bars. I have only toured like this in rich first-world places, where you are never far from a hot shower and a comfy bed wherever you go, but if you reckon you're hard enough, read Journey to the Centre of the Earth by Richard and Nicholas Crane for a remarkable account of this style of travel through some of the most isolated regions of the world.
The advantage of all this is that with such small loads the bike handles as well as it does unladen, you can actually climb worth a damn, and it still accelerates to speeds greater than 12 mph, so you still get that cool, roadie "whoosh" experience whilst enjoying the freedom of exploring by bike. The lowest gear on my racing bike is 40/26 (~41"), which is even adequate to get over the Lakeland passes (up to 33 %) carrying this kind of tiny load.
For luggage, there is nothing to beat the traditional saddlebag. It needs no rack, so the bike can be set up with racing-style minimalism normally. Just sling the bag on at the last minute, the straps going round the seatpin and the saddlerails. I use a Carradice saddlebag which is very "Old School", but it works. It is made of something called "cotton duck", which claims to be waterproof because as it gets rained on, the fibres swell and prevent any further entry of water into the bag. I admit this sounds like bollocks, yet it appears to be true. A saddlebag also has the advantage that it puts the weight at the centre of gravity of the bike-and-rider combination: exactly where it should be to have a minimal effect on handling, climbing and so on. You never know it's there. I also use a handlebar bag when on this type of tour. This is because I like to have a proper camera with me (an SLR) which won't fit into a jersey pocket and to which I need quick access because otherwise I won't be bothered using it. Providing the handlebar bag isn't too laden though (basically I just keep my camera, keys and wallet in it) it has a minimal effect on the behaviour of the bike. Although the bag touches the cables, it does not appear to impede gear shifting with Shimano STI-style combined brake levers/gear shifters.
A suggested kit list for ultralight touring is below. With this you should be able to travel in a comfortable and civilised fashion fairly indefinitely, at least in Europe or America. Some notes: The toolkit is what I take, using a Shimano 105-equipped road racing bike. You might need different stuff. But the assumption is that any really big jobs (e.g. bottom bracket removal) wouldn't need to be done on a shortish tour in a region with bike shops dotted about so you might as well just carry the stuff needed to keep you on the road. Shower gel is good for washing everything-I wash shorts, socks and jersey in my room's washbasin with it and dry them overnight. There's a spare set for use if they haven't quite dried by the following morning, but they usually do. If it's just the sort of trip where we are off for a long weekend, taking the train out to the middle of nowhere, riding for a few days and then getting back to a train station to return home, I don't usually bother with shaving. For a change of normal clothes for dossing about off the bike, nylon or polyester traveller's gear is brilliant, being light, quick-drying, packing up very small and crumple-proof. I have Time racing shoes with external cleats that you can't walk in on my racing bike, and they also let the rain in. This is why I need the neoprene overshoes and a pair of trainers for the evenings, but with Shimano SPD pedals or toeclips and straps and proper waterproof shoes, you can get away with only one pair of shoes. The sheet sleeping bag is good to have for gites in France, but in the UK now it seems that even most youth hostels always provide bedding for no extra charge. Silk ones are about a quarter the weight and bulk of cotton ones.