1) The fact that UAC design assumes that every setup executable should be run elevated (and that a user doesn't really have a choice to run it from a non-elevated account),
2) The fact that UAC implementation contains bug(s), like e.g. the bug I pointed out in my article, which allows a low integrity level process to send WM_KEYDOWN messages to a command prompt window running at high integrity level.
I was pissed off not because of #1, but because Microsoft employee - Mark Russinovich - declared that all implementation bugs in UAC are not to be considered as security bugs.
True, I also don't like the fact that UAC forces users to run every setup program with elevated privileges (fact #1), but I can understand such a design decision (as being a compromise between usability and security) and this was not the reason why I wrote "The Joke Post".