I will preface this by saying this is just one person's opinion and your mileage may definitely vary...
I am inclined to say that pretty much all touring items have a minimum level of serviceability that once met the benefit of spending more is not that great. I think that there is a tendency to go way overboard when folks list what they need on many of the items with the bike itself, the racks, and panniers being especially likely to fall into that category.
I would say that from the standpoint of actually affecting the quality of the experience, the equipment you use has very little effect on whether the tour was a positive experience unless it is really inadequate. The area where gear choices have the biggest impact on in the experience itself is comfort, but in all but a few items comfort is best attained by carrying less weight, not by carrying more comfort items.
I don't think that my tours to date would have been substantially different if I were riding a $6000 bike instead of a $600 one. So for me the list is something like:
Items where you only need to meet a minimum standard of suitability:
- Pretty much everything about the bike itself. I think the minimum standard is much lower than many seem to think. Deore and Tiagra components are fine.
- Racks. They need to be sturdy enough, but relatively inexpensive ones are fine. For example Blackburn EX-1 rear rack and lowrider front rack clones from Nashbar or Performance are a good choice. The worst case IMO is to spend a lot and still get heavy ones.
- Panniers. Nashbar or Performance waterproof ones are fine and are among the cheapest (and also lightest).
- Sleeping bag in moderate weather. A $70 Slumberjack was fine for me on the TA and many other trips including non-cycling trips.
- Clothes except maybe shorts. They are kind of on the line between categories. Even shorts I only get moderately high priced ones (Pearl Izumi Ultra Sensor)
Items where spending a bit more is worth it to me (note that even in this category most of the items I pick are not extremely high end):
- Sleeping pad. If it saves weight and adds comfort (My NeoAir did both and assuming it proves to be durable it was one of my best purchases.
- Sleeping bag if the weather is more severe.
- Tent. If it saves substantial weight.
- Food. If it makes the trip more enjoyable. Definitely sample the local foods. This does not need to be a big splurge, but don't be too chintzy either.
- Shoes. It is worth buying Sidis to me, but I like the lower end Bullet 2's so it isn't a big splurge.