The answer lies in quantifying the project impact, not in calling it low/medium/high (which is a subjective, relative term). Also, as business grows (or shrinks), the measurement of impact should be weighted as well. For example, a project that generates $1M/yr in revenue is a big deal when you're making $2M/yr, but not as much when you're making $20M/yr.
In the end, limited resources need to be focused on the area where it makes the most impact rather than trying to solve everyone's problems. That is exactly what IT management's job is.
The other answer is that no group/team/company does this really well, it comes down to individual manager's or IC's style and how you dismiss the trivial requests.