The Concentration Game

The issue of "concentration" for spellcasting has come up a number of times in my OD&D games lately. For those somewhat more familiar with the current-edition of D&D, two reactions are halfway common: (a) surprise that a "concentration" mechanic existed in OD&D, and/or (b) surprise that it's so infrequently required for spells in OD&D.

At any rate: Yes, Virginia, concentration was a requirement for certain spells in the very first publication of D&D (1974). Just to reflect on it a bit more, here are all the spells which mention such a requirement:

Original D&D

• Phantasmal Forces: "As long as the caster concentrates on the spell, the illusion will continue unless touched by some living creature, so there is no limit on duration, per se". This is the first appearance; kind of interesting it's used as means to avoid giving the spell a defined duration.
  
  
• Wall of Fire: "The spell will create a wall of fire which lasts until the Magic-User no longer concentrates to maintain it." This is one of my least favorite uses (in that it locks the caster down to nothing but maintaining the wall while it's in use -- can't even flee while the wall serves as an obstacle). Requiring concentration while the caster directs some dynamic activity makes sense; just maintaining a static effect not so much.
  
  
• Conjure Elemental: "The Elemental will remain until dispelled, but the Magic-User must concentrate on control or the elemental will turn upon its conjurer and attack him (see CHAINMAIL)." This is likely the most famous, idiomatic usage: concentrate on your summoned elemental or it irremediably go berserk and tries to kill you instead (which is a major downside to this spell!)

And that's it: just those 3 in the little brown books (Volume 1, Men & Magic).

A few other references can be found in Sup-III, Eldritch Wizardry, for the psionic domination ability; and in Sup-IV, Gods Demigods and Heroes, in that the Hyborean magical mirror of Lazbekri requires it (so as to function as a gem of seeing; maybe just flavor text), and that the Indian deity Rudra can cast illusions without such a requirement (as is normally the case for phantasmal forces, above).

So that's a very slight number of uses of the mechanic -- and as usual for the rules in those days, didn't really scope out exactly what the parameters of such a concentration requirement were.

The AD&D rules somewhat expand on those points, including requirements for -- gnome and halfling detection abilities, paladin detection of evil, spiritual hammer, locate animals, locate plants, wall of fire, conjure elementals, clairaudience, phantasmal forces, telekinesis, emotion, a few closely related spells, and numerous psionic abilities (in the optional Appendix I). 

One interesting point is that the 1E phantasmal force specifies that concentration ends with any movement (in addition to taking an attack, etc.), whereas the new improved phantasmal force permits half-speed movement. I find that latter rule pretty reasonable, and use it as a base assumption in my games.

Furthermore, in my Book of Spells (see sidebar) I have a tendency to include it any spells that have an ongoing detection or direction component, including (in addition to the classic ones above) -- read minds (ESP), legend lore, and move earth.

Of course, 5E D&D expanded the concentration mechanic to be a core limiting factoring in a lot more spells -- maybe something like half of all the spells in the game? When playing a caster in 5E, I routinely get tripped up by not foreseeing how many different spells have the concentration requirement, and wind up boxing myself out from good combinations and/or embarrassingly wasting a number of castings.

What's your master theory for what spells should require concentration?