You can change multiple text layers at once: select all layers – activate Text tool, but don’t click on any one text – change font/size/color/etc. in the tool-bar.

Shortcuts for Layer-Blend-Modes: Shift + Alt + M = Multiply (or: N = normal, O = Overlay, G = lighten, +/- on the numpad cycles through all of them)

If you’re making a selection (using the Rectangular Marquee Tool, for example) you can press and hold Space (all the while not letting go of the Mouse-Button) to move the selection. (thanks to Helga)