Protection from Evil: This spell hedges the conjurer round with a magic circle to keep out attacks from enchanted monsters. It also serves as an "armor" from various evil attacks, adding a + 1 to all saving throws and taking a -1 from hit dice of evil opponents. (Note that this spell is not cumulative in effect with magic armor and rings, although it will continue to keep out enchanted monsters.) Duration: 6 turns.The description doesn't say "Chaos" at all, and the Magic-User version of the spell isn't reversible, although the Cleric version is (More on that much later.) There is no "Protection from Law" or "Protection from Good". I'll cover that more when I get to Detect Evil.
More importantly, the very first listed feature of the spell is that it keeps out attacks from enchanted monsters. That's the whole point of the spell, but it's a feature that gets overlooked or even dropped later on. The spell also specifies that it protects the conjurer, not anyone else. Although I'm inclined to loosen that restriction so that the spell can be cast on other people, it does imply that the intended use is to protect the spell caster from supernatural attack long enough to cast some other spells.
In fact, the six-turn duration could be interpreted as ten-minute turns (one hour total) or one-minute combat turns. Based on the supposition that it's a delaying tactic, like Hold Portal, I'll go with combat turns. I'd like to tentatively stick to the combat turn interpretation as long as possible for all spells to see if it alters magic too much or is actually a viable simplification.
Before I get too far into a discussion of "enchanted monsters", I'll mention the other feature of the spell, the saving throw or defensive bonus. The description specifically mentions that it does not stack with any other saving throw or defensive bonus. Since there's also a comment in Monsters & Treasure that magic armor and magic shield bonuses don't stack, either, this may be another tentative broad-reaching rule: use the best defense applicable, not the total of all defenses. This leads to much lower numbers, less futzing around with calculations, and a lower-power feel to the game in general.
The spell is still useful if wearing magic armor, though, because the primary purpose, again, is to keep enchanted monsters from attacking the protected conjurer. But what's an enchanted monster? It's never defined in the early rules, which I take to mean that we should be using the common understanding of the words (in other words, what a non-gamer, non-fantasy fan would think.) An enchanted monster would be a monster created by, summoned by, or altered by enchantment. It's not necessarily a spell-casting monster, but an inherent magical ability would probably qualify. There's going to be a lot of leeway from GM to GM, depending on what feels right for the campaign; for example, if a dragon's breath weapon is defined in biological terms, as some GMs or fantasy writers define it, then a dragon isn't an enchanted monster.
My personal preference is to define everything on page 1 of the Monster Reference table from skeletons down through lycanthropes, but excluding manticoras, hydras, and wyverns, and treants, invisible stalkers, elementals, djinn, and efreet on the second page.