Those brush strokes are really nice and expressive.
In Illy CS5 the edges of brush strokes (that is corner points) can look significantly better. Maybe you can play with that setting in your brush: it's called "Overlap".
But it looks like the scaling behaviour of your stroke is also different. Since "Stretch between guides" has been introduced in CS5 it might have affected the whole scaling behaviour of brushes.
Have you adjusted the scaling of your brush in the CS4-file with the "Options of selected object" button? Maybe the setting you have made in those options gets somehow set back to 100%.
If I change the Options of selected object for one particular stroke in CS4, the same one I showed in the screen shot, here is what happens. The specifications were for it to be 1.7 points. I had manually specified .3 point in the path stroke settings. If I make the specifications .3 point, the stroke loses all its character and is barely visible. I think the expressiveness of the stroke may have to do with the fact that I made this manual tweak.
Also, I have SEVERAL THOUSAND such strokes in this particular file, and they were based on a variety of brush designs and were all manually overriden in this way (not one at a time though; I just kept drawing with the manual override and it maintained it).
I cannot imagine how many weeks it would take me to revisit them and tweak them to fit new brush behaviors (if this is not a CS5 bug). I really want to take this and other files into CS5 because the performance of everything else is so much better.
I hope someone will chime in who knows what's going on between CS4 and CS5 in this regard and has a solution. I don't want to resort to using Expand Appearance or Outline Paths to solve this - I want to maintain their integrity as brush strokes at least until I am completely done drawing...
Bill, opening an CS4 brush and only modifying the strokes value shall not give such redrawing. If you can can you send me such CS4 document and the same edited in CS5 showing the behavior. I don’t guarantee I will find what is going on, but maybe. It may take a few days before I reply to you has i’m the middle of some urgent task. See my profile for contacting me.