My two cents on flat sprints (no hurdles or steeplechase):
40yd/50m/55m/60m: All about your reaction time and acceleration; you may only get close to your top speed at the very end. I suppose whoever wins these short sprints can be deemed the "quickest man/woman in the world" based on pure reaction time and acceleration.
100m: Reaction time and acceleration still important, but you can get to your top speed toward the end. Therefore, whoever has the fastest 100m time (so long as they still have decent reaction times and accelerations) is deemed the fastest man/woman in the world, as their top speed beats out the quick starters that would win in the 40yd/50m/55m/60m.
200m: All about maintaining your top speed over a longer distance.
300m/400m: Too long to maintain your top speed, but short enough that they are at a threshold just below your top speed. These are the lowest end of speed endurance and the longest traditional sprints. In these races, however, speed is far more of a component than endurance.
500m/600m: These are too long to be a traditional sprint, and are thus the shortest middle distance events. Sort of a transitional area in my opinion; lower end of speed endurance. Speed is still more important than endurance here, but less so than in the 300/400.
800m: Pure, intense speed endurance. Speed is important, but just because you run a 45 second 400m does not mean you can run a 1:45 800 without endurance. Likewise, endurance is important, but running a 3:50 mile or 13:00 5K does not guarantee a 1:45 800 if you don't have a fast 400m. You must have developed both endurance and speed equally (thus, speed endurance).
1000m: Similar to the 800m, but endurance is slightly more important as the race is slightly longer.
1200m: Just a tad longer than the 1000m, so endurance gets more important than speed.
1500m/mile: Higher end of speed endurance (4ish laps). Still speed endurance because WR pace equates to 54 high for the 1500 and 55 mid for the mile. Here, though, your endurance ability is more important than your speed ability.
2000m: Endurance continues to grow in importance, but pace is still quite fast.
3000m/two mile: Highest end of speed endurance. WR pace is sub 60 per 400m but you are enduring this pace for 7-8 laps around the track. Endurance ability far outweighs your speed ability, but of course you still need basic speed to run 7:17 or 7:54.
Above this, you have moved into distance running. WR 5K pace is still quite fast, but not in speed endurance anymore.
Note: "Speed endurance" is a transition between the pure speed needed in the short sprints, and the pure endurance seen in the long distance events. This transition is the middle distance running events from 500m to two miles. Below 500m, traditional sprints; above two miles, distance running.