Much of what happens in an old-school role-playing game is (or should be) decision-making, exploration, and/or role-playing, in no particular order. Entire playing sessions might go by without a single die rolled or sword drawn from its scabbard, and these are often the best because now it's 100 percent about the participants and what they can dream up. This aspect of the game must be preserved even when rules are otherwise necessary, and make no mistake, rules will be needed...
Rules are necessary because they (1) quantify the powers and abilities of characters, monsters, and magic items and (2) resolve situations where the outcome is uncertain. This is a good thing, and as old-school enthusiasts we sometimes fail to praise rules and what they do for us. However, rules can often overstep their boundaries and become a substitute for individual decision-making and problem solving to the detriment of play.
Why does this happen? Laziness is one, somewhat uncharitable, possibility, but it can be something else. Despite a player's best efforts, some uncertainty still remains and, over time, the group starts skipping the action part and just rolls for the outcome, making players mere spectators in their own games. The right set of rules can help immensely, and the referee should have no problem finding something good - shop around!
The other solution is to tie uncertainty to actions taken by the players. For instance, imagine the party comes upon a pile of rubbish that the players wish to search for valuables. If they prod it with a ten-foot pole, their chance of getting bitten by the poisonous spiders within falls to zero. But if they dig in using their bare hands, this chance goes up dramatically, often with fatal results! Either way, players are required to make decisions and describe their actions and, each time, the level of uncertainty is tied to previous choices made in the game.
In role-play, we think in terms of fixed difficulty modified by ability scores and the use of magic items. However, we should also be thinking about uncertainty as decided by player actions in advance of a particular situation, and right action can sometimes eliminate uncertainty (and related dice) completely, or at least favorably modify the attempt. This is the most direct power characters should ever have over their own fates, and referees can use this approach to encourage and reward good choices.
In other words, make the players describe their actions, devise solutions, and act out interactions, and then base any difficulty on what they did and how well they did it. This will give them the challenging adventures they long for, secure in the knowledge that good decisions are worth making and well-rewarded!