(Probably related to this one, and probably should be CW.)
A very long time ago, I had the good fortune to read George Gamow's excellent series of Mr. Tompkins books. That introduced me to the idea of a world where the usual physical constants (e.g. the speed of light and Planck's constant) were changed such that "paradoxical" effects became apparent in the macroworld.
My memory is hazy now, but I do recall the concepts of relativity (e.g. dilation) becoming more pronounced when the speed of light is reduced to "human-sized" speeds.
In this vein, I ask this: assuming all other physical constants being fixed, what exactly can be expected to happen if (physical constant of your choice) is increased/or decreased?
One physical constant per answer, please.