Some are pretty easy like non-smoking or medical needs (no stairs, H/C, close in, near elevators, etc), or broad view categories (pool, lake, golf course, woods, etc) or remote area for a more quiet stay.
With the newer resorts adding more H/C accessible rooms, some are now requesting no H/C because of some concern or other. Same with a dedicated 2 bedroom as opposed to a lock-off.
The challenges usually begin with requests for specific buildings, floors, or room numbers. My opinion is that is expecting more than is reasonable.