paranoid marvin has it about right. There's a distinction to be made between donations and taxes. We don't really see regular taxation until the late Middle Ages. First in cities (there's the famous catasto of 1427 in Florence ... though I guess saying "famous" somewhat marks me as a historian <g>). A city provides a limited citizenry--you can keep rolls--and limited geography. So it becomes possible to staff the enterprise.
Much harder to do that for a kingdom. The early examples come from France (maybe England; I'm on surer ground on the Continent) with the salt tax (the gabelle). Here again, you aren't having to deal with the entire kingdom. You can control (somewhat) points of entry and points of sale, enabling a tax to be levied, though it's really closer to a toll, which has a longer history.
An unexpected consequence of general taxation was that it gave the general population a point of protest. You start to see peasant rebellions at the end of the Middle Ages (starting with some famous ones in mid-14thc, but those were special circumstances).
As for the upper nobility, that gets really complicated, but the easy version is this: privilege.
It's a great word. Compounded from "private" and "law". The more powerful and connected you were, the more exemptions and special treatments you could get. This included simple dodges such as moving possessions out of the house when the assessor came around (Italian cities were great at this), hiding possession of real property (medieval equivalent of off-shore accounts), but extending to getting laws passed that specifically exempted whole groups of nobles. One of the real complaints of the wealthy merchant class in the LMA and early modern eras was exactly that: the nobles had privileges they did not. For the most part they couldn't win that war, so they just married into the noble class. Lucky for me, tough for you, mate!
I've said it before but it bears repeating: the Middle Ages were *way* more complex than modern times. When people talk about how complicated modern life is, it's always good for a chortle.