Strokes are often brushed off because witnesses (and victims themselves) aren't aware of the warning signs. This is what happened to my paternal grandfather, and the results were permanently disabling. The damage would have been reversable had it been detected earlier.
Bystanders can now recognize a stroke by asking three simple questions of someone who is displaying symptoms (loss of coordination, slower than normal verbalization or thinking, etc.). The acronym STR for "STRoke" will help you remember:
Ask the person to SMILE.
Ask them to TALK and form a simple sentence (i.e. "It is warm out today.")
Ask them to RAISE both arms to shoulder height.
If he or she has trouble with any one of these tasks, call 9-1-1 immediately and describe the symptoms to the dispatcher.
Here is a more comprehensive list of stroke warning signs from the American Heart Association.
(Hat tip to: Carl Fredericks)