Show mana regen in minutes
Let's be honest here; 0.04 per second doesn't really represent any meaningful amount in your head at first glance. I have to get out the calculator and multiply it by 60 before I can go, "Oh, I see, so -that's- how long it will take to cast another spell."
The display would be a lot more helpful if it showed, say, 2.4/m instead of 0.04/s. One sideglance and you instantly understand how fast it's really going.
This also allows for a higher level of detail, e.g. when it hits 0.045/s you normally still see 0.04. This way, instead you'd see 2.7/m regen. The effect is even larger if two decimal places are still shown.
Of course, this can always be a toggle-able option, as well.