Allow users to set the priority of a task and also number them by order to be completed.
I'd love to look at my daily calendar and see my tasks organized by priority and the order they need to be done. For example, "High Priority" tasks could be marked with an "H," "Normal Priority" with a "N," and "Low Priority" with a "L." Then I could go through and number them, so on my calendar, it appears like:
H1 - Complete report for Dave
H2 - Contact client about budget update
N1 - Finish mission statement example
L1 - Compile list of possible caterers for upcoming event
Each day you could see what needs to be done first, rather than what comes first in the alphabet.