We've all noticed by now that D&D rewards more experience for treasure than it does for slaying monsters, which means that wise players change their strategy to take treasure without getting into a fight. It's secret encouragement to focus more on the treasure. Not so secret anymore, of course, because it's been discussed so often.
We've also all noticed that Charm Person is more effective on your first or second adventure than mere Sleep or Magic Missile. Aimed at the right person, it gets you a better ally than you could hire in town and effectively doubles your combat ability... for an extended period of time. Played right, that single Charm Person spell will kill more monsters than either of the other two. It's secret encouragement to think in terms of building a group instead of going it alone.
In the LBBs, there's another bit of secret encouragement: short-range missile weapons are at +2 to hit, and the only attack bonus for high ability scores is the Dex bonus on ranged attacks. That means you are more likely to hit with a ranged weapon in the dungeon than in mêlée. Even with a thrown dagger. Which, in turn, means that wise players will try to arrange attacks from a distance as much as possible.
What other forms of secret encouragement are buried in the OD&D rules?