When the quantum character of proton transfer is taken into account, the intrinsic slowness of self-exchange proton transfer at carbon appears as a result of its nonadiabatic character as opposed to the adiabatic character of proton transfer at oxygen and nitrogen. This difference is caused by the lesser polarity of C-H bonds as compared to that of O-H and N-H bonds. Besides solvent and heavy-atom intramolecular reorganizations, the kinetics of the reaction are consequently governed at the level of a pre-exponential term by proton tunneling through the barrier. These contrasting behaviors are illustrated by an analysis of the CH3H + -CH3, H2O + OH-, and +NH4 + NH3 self-exchange reactions. The effect of electron-withdrawing substituents and the case of cation radicals are discussed within the same framework taking the O2NCH2H + CH2=NO2- and +.H2NCH2H + (CH2NH2)-C-. as examples. Illustrated by the CH2=CH-CH2H + -CH2-CH=CH2 couple, it is shown that the "imbalanced character of the transition state" is related to heavy-atom intramolecular reorganization. Combination of these various effects is finally analyzed, taking the O2N-CH2=CH-CH2H + CH2=CH-CH=NO2- and +.H2N-CH2=CH-CH2H + (CH2)-C-.-CH=CH2-NH2 couples as examples.