The BIGGEST difference is in the GUARD PLAY of the purples compared to the blues. Of course that isn't all....
The purple belt has more TIME in. That's what makes him generally better than the blue in most aspects. This added experience gives the purple more attributes such as better timing and sensitivity.
The purple's technical command will be greater as well. And even though the purple may not know a great many more techniques than the blue, the added experience helps him to better apply the techniques when wrestling.
Perhaps one of the main differences is that the purple is usually much better at COMBINING techniques. He's more able to flow between them. So if one should fail or is countered, he is already onto the next move in the chain. He can also think about more than one move at a time. In other words, his ability to create strategy is better. He can think several moves ahead and set up certain things which the blue is often not able to do as well.
The amount of overall strength used in rolling is also likely to be diminished as a purple because of the higher technical command. Thus he will be able to roll longer and not tire as easily. This is often coupled with a better ability to RELAX on the mat - something that a LOT of people have difficulty doing.
The purple's defensive skills are often more highly developed. Hell, everything is. But the purples timing out of guard passes is usually so much better than blues, that they are often able to more easily recover guard without the passing happening as frequently (as it happens to blues). If the pass DOES occur, the purples are usually working more transitional defense and don't seem to get caught as much flat on their backs with their arms killed.
Lets just say that purples know about the same things (a little more usually though) as blues, but they just better at it. Their timing and mat experience is just that much better.
Gotta have your escapes down so you don't have to think about them.
Gotta have the ability to play closed and open guard, and to be able to RECOVER the guard and prevent passes.
Again, I have to say that the best thing about purples is that their GUARD play is usually MUCH better than the blues. They're beginning to develop the impassable guard (theory).
With all this said, it's completely subjective and depends on many factors such as, prior experience, athleticism, wrestling or football background (if applicable), conditioning, sandbagging, etc.
Here's a link you may find helpful:Harris Progression
I'm sure that Fletch will add some insight to this as well. Nice thread!