Panniers should be durable and waterproof. I don't need a lot of pockets, but some people like them. I prefer not to mess with rain covers, but some panniers need them. Pack so that you have some spare room for when you want to add extra fluid and food. You can get them anywhere from almost free (make them yourself from kitty-litter buckets) to very expensive. Make sure they have a good attachment system. I typically leave my panniers on the bike at all times, but if you like to bring them in at night, get ones that go on and off easily. Ortlieb and Arlel are the two high-quality (and high price) brands, but others will work too.
As Pat says, flats are the #1 repair item. A FiberFix will allow you to temporarily fix most broken spokes, but I also like to carry a hypercracker (Stein Mini Lock Ring Tool), a few spare spokes of each size, a few spare nipples and a spoke wrench. You should also carry the most common Allen wrenches (4, 5 and 6), primarily for adjusting brake cables as the pads wear, and for manipulating your seat post. I like to take a spare set of brake pads, especially for long trips where your pads might wear out. I also carry one spare cable of each type, although I have never used them. Start with fairly new tires, but if it's a very long trip, you might consider taking a spare tire. Some people do, some don't. Sometimes tire problems come on with very little warning.
Everybody has their favorite tire. I like Schwalbe Marathon, and am currently running the Marathon Mondial folding tire (not the wire bead version of this same tire). Many other tires are also perfectly fine. Selecting the right width is as important as selecting the right brand. If your tour is mostly on pavement, I like 35mm width.
Saddles are very personal. What one person loves, some other person hates. There is no substitute for many, many hours on the bike to know if you like a saddle or not. I use the B-17 Pat mentions, but not everybody likes it.
I like a two-person tent for extra roominess. A two-person tent usually weights about 50% more than a claustrophobic one-person tent. I like a floor, for bug protection and extra rain protection, but you can save a ton of weight by skipping it. Lightweight is usually better than heavy, but lightweight frequently means less durable and much more expensive.