I can tell you exactly what it is. Inside the doors MB put some "sound deadening" material. Basically, they stuck a few small (1 foot by 6 inches) pieces of tar paper with this nasty brown goo on it inside the doors. After a few years, they start to curl, and once they curl enough, they touch the window glass. That is where the goo is coming from. To fix it, pull of the interior door panel, and pull out the tar paper sheet. If you want to replace the sound deadening, use something like Dynamat which you can get at a car stereo store. It works (and sticks) much better.

To get the stuff off you glass, here is how I did it. I scrapped off as much as I could with a razor blade. It will leave some behind. I then cleaned it with a solvent (I used brake cleaner, but you can also use nail polish remover, just try not to get it on the Paint!!!). A small amount of residue wil probably remain. I used fine steel wool to buff it off. Get the remaining fingerprints off with windex, and you are good as new.