If you happen to have a Galaxy Note 2, or are planning to purchase a Galaxy Note 2 and have any questions about which USB devices will work when plugged in — this video will probably give you the answer. The video, which we can only describe as being epic, is 50 minutes long and shows everything from game controllers, to keyboards and mice, to USB hard drives, to memory card readers and much more being connected.
Anyway, while the video is on the longer side, there are lots of time codes and shortcuts listed, so for some, you can probably avoid having to watch the whole video. For example, those curious about a USB keyboard with a trackpad can skip to the 5:34 mark and see how the keyboard works and the trackpad even has support for multitouch gestures.
Otherwise, while the vast majority of the items do not require root in order to work, there were a few that did. Those items were clearly noted in the video. And for some, the usefulness may vary. For example, a 2TB external hard drive was recognized, however it needed to be plugged into external power and took some time before the files were viewable.
Bottom line here, while a wide variety of devices are able to be connected, the usefulness and amount of before-hand preparation seem to vary.