A more negative value of E°' is associated with a greater tendency for the oxidized form to take electrons in a redox process, and a greater tendency for the reduced form to give electrons in a redox process. For example there is another redox pair, namely ubiquinone and ubiquinol, and E°' is roughly +0.06 for the reduction of ubiquinone (I have seen similar but not identical values, depending on the source of information, and there is no need to worry about small differences at this point). Which is the stronger oxidant, NAD or ubiquinone? Which is the stronger reductant, NADH or ubiquinol?
Many nitrogen-containing species have a formal charge of +1 on nitrogen yet all atoms have filled shells, for example NH4+
. I suggest looking at the structures of NAD and NADH, preferably with the electron pairs drawn out explicitly. Wikipedia's drawings do not show the lone pairs, but they can be added in. In both structures the nitrogen has a full octet. What is different about the nitrogen in one versus the other? What is different about the two six-membered rings?https://en.wikipedia.org/wiki/Nicotinamide_adenine_dinucleotide
I think that it is probably better to keep separate the concepts of electrophilicity and ability to act as an oxidant. I have a couple of reasons for saying so, one of which is the nucleophilicity and electrophilicity are defined in kinetic terms (how quickly something reacts), whereas E°' is a thermodynamic quantity.