--Some questions for Miles...
-- how reliable is it to find particular animals in any one spot? Or rather, how nomadic are they?
Some are very reliable..
You will see bison in Lamar and Hayden Valleys... guaranteed.
You will see elk near mammoth, I didn't even put that site on the map.
Some are very hard to spot, ermine and pine martens come to mind.
Some are moderately reliable, bighorn sheep near gardiner for example.
You need to network with the other wildlife types when in the park. They will always have the latest scoop.
Don't be shy, 98% of the serious photographers are willing to help you.
- a guide I was looking at said to look out for Bighorn Sheep on the trail up to Washburn and indeed, find me they did to the point where being caught up trying to work out which/how/what to photograph, I got surrounded (not something I'd recommend anyone try to replicate but they really didn't seem to care that I was there!) Are they likely from some other area or is the park so full of animals that don't really care about boundaries that they can be anywhere?
It's mostly all about food. In the wild animals have to work hard for their supper.
... no mention of moose. Is this an indicator that they're not reliably found or seen in the park?
Moose are not reliably seen Yellowstone.
IMO the Gros Ventres river in Grand Teton NP is a much better bet or Pahaska Teepee, just outside the east entrance to Yellowstone.
-- In the list of "water features", how do you decide what to include and not include? For example "Morning Glory" (a ladder - and not a plane - should be sufficient for that one!) and the "Norris Geyser Basin."
I'm not very found of the geysers. The Norris Geyser Basin is IMO not very photogenic. The lower geyser basin would be a better bet if you like geysers and thermal pools. Many of the other thermal features are ugly brown and don't do much for me. If you want to carry a ladder 1.2 miles on the Fairy Geyser trail, be my guest.