User-friendliness is what really matters

on space_check(threshold_percentage)
    tell application "Finder"
        set the percent_free to ¬
            (((the free space of the startup disk) / ¬
                (the capacity of the startup disk)) * 100) div 1
    end tell
    if the percent_free is less than the threshold_percentage then
        tell application (path to frontmost application as text)
            display dialog "The startup disk has only " & the percent_free & ¬
                " percent of its capacity available." & return & return & ¬
                "Should this script continue?" with icon 1
        end tell
    end if
end space_check

>>1
Actually, true user-friendliness would be Automator.app.
Unfortunately it doesn't seem to be as powerful as Applescript. I struggle to find a way to reproduce your script in it, maybe it's just my unfamiliarity with the IDE: http://img19.imageshack.us/img19/8319/picture1nhu.png