Let me see how many paths are in my document once all the appearances are processed.
I do a lot of crazy things with the Appearance palette. Sometimes it's really easy for me to start generating absolutely apocalyptic numbers of paths without realizing it, and make Illustrator start acting really weird in seemingly unrelated places.
I would love it if the preview renderer (both cpu and gpu!) kept track of how many paths it is actually rendering once all the effects on all the paths in the documents are handled, and alerted me when I start getting into catastrophically huge numbers of paths. The threshold where this starts happening should be user-configurable, so it can be tuned for the limitations of my current computer.
Also it would be nice to have this number available in the Objects section of the Document Info window next to the number of paths. I shouldn't have to expand every appearance in my document to discover that its 2.4k paths turn into about 10.5k paths once all the effects and blends and brushes and whatnot are processed.