It seems that when you apply the Dynamesh tool to your model, it bases the polygons (actually voxels, to be specific) on the lowest subdivision level of your model. This is easily worked around by simply starting off with a Dynameshed model (ie., just hit Dynamesh the moment you load your basemesh up), but then you come across the problem of not being able to return to the lower subdivisions for later tweaks. In effect, once you Dynamesh the model, any sculpting you applied beforehand is now baked down into the model.
Granted, you can change the number of "subdivisions" when applying the Dynamesh tool, so in effect you can subdivide/re-mesh by a small amount to keep vertex counts low, but still-- if you want to go back down to the super-low-polycount basemesh for extra tweaking, it seems you're kind of SOL.
All that said, I only just discovered the Clay Polish tool, right above Dynamesh on the toolbar. Now this tool is awesome! It essentially smooths over the entire mesh while maintaining all the creases and edges that you initially detailed in.
Below is an example:
For a quick study and a means to play around with the new tools in ZB4R2, I modeled a bust of Leandro Fernandez's take on Paul Crocker, from Greg Rucka's Queen & Country comics.
As you might be able to see around the eyes and necktie, there are quite a few stretched polygons that show up as jagged pits rather than smooth lines. I could subdivide the mesh to add polygons and smooth it over, but I'd have to subdivide 2 or 3 more times to get enough density to make a smooth line, which would bog down my system and add polygons in areas that obviously don't need them.
Instead, I messed with the settings of the Clay Polish tool, and the result was great!
|Clay Polish is like 3D Rogaine!|
As you can see from the final model, jaggies are gone, edges are cleaner, and he has hair now!
Okay, fine-- I added the hair afterwords, and I'll admit that some of the shallower lines, like those down his cheeks and forehead, were mostly lost and I had to redraw them. But I bet with a bit more tweaking of the Clay Polish settings, I probably wouldn't even have to do that.
Of course, this rather pales in comparison to what you could potentially do with Dynamesh, as far as retopology. But it's still a great means to smooth out the majority of a mesh without losing all of your detail.