(and yes, this in many ways is the root cause of the current world financial problem, but just saying)
() Discretionary bonuses, that is, where a human decides what you get, rather than a fixed rule.
A fair question to ask when negotiating would be what % of the bonuses are currently paid, and when was the last bonus cut. Might give some indication of the reliability.
A salary of 50K is a salary of 50K, you get 50K over a year.
How does that compare to a salary of 45K + bonus? Is it more, less, or the same?
When I'm comparing, I use only money I'm guaranteed: salary plus any guaranteed or performance bonuses I am confident I can hit (with targets in writing as part of my job agreement). Bonuses that are based on nomination or any other "approval" type process are really just too risky for me to consider as part of my overall compensation package. They will only factor if all else is basically equal.
Is it possible to make more through total compensation of lower salary + bonus than high salary + little to no bonus? Sure. But I'm happier with the salary, and my hunch is that many other employees would feel the same.
A company with a great benefits program (ex. paying full tuition on a degree) can drop it from one year to the next.
So if you have a budget that includes that degree being paid for, you might find yourself suddenly strapped for cash when you are footing the bill for the degree yourself.
Anecdote: The company I work for had a particularly rough quarter years ago and couldn't make bonuses. Those that had planned on their bonuses being there were suddenly left with a deficit. Even though the company increased bonuses next quarter to make up, that didn't help those guys then.