A Monty Haul game is one in which the dungeon master gives out large amounts of treasure and magical items with very little effort on the players part. This is the sort of game where everyone has a magic sword well before they reach second level. Some Monty Haul games go so far as to have characters who have ability scores over the normal human limit, carry multiple artifacts, and generally wield all the power of a god (but with none of the responsibility).
From the
AD&D second edition's
Dungeon Master's Guide
At the other extreme, the problems of too much treasure are not so easily solved. Here players may enjoy the game and why not? Their characters are doing quite well. They have sufficient money and magic to best any situation the DM can devise.
However, the DM seldom has the same enjoyment. He is faced with the task of topping the last lucrative adventure. He must make each adventure a greater challenge than the last. While this is true for all DMs, it is grossly exaggerated for the DM who has given out too much: How do you top the adventure where the fighter got the Hammer of Thor or some equally valuable item?
Invariably, the players reach a point where they, too, become frustrated. Everything is the same"Oh, we did this before," or "Ho-hum. Another Sword of Instant Monster Destruction." Soon there are no challenges left, because the characters have earned everything in the book!
A dungeon master must be extremely careful not to let his game evolve into this sort of challengeless crap fest. If characters begin to become to rich, simply find a way to take some of it away from them (tax collectors, bandits, and natural disasters work well). Avoid the pitfall of swapping around magical items that appear in modules, to the ones the player can use (don't change the Spear +2 into a mace, just because none of the players can use a spear). Don't go out of your way to make items fall into a players lap (there is no way the players are going to spot the one magic sword among the forty carried by the attacking orcs, unless someone casts a Detect Magic spell).
No one should have more than a half dozen permanent magic items (usually less, usually much less). The 7th level NPCs that the players meet are not carrying 18 individual magic items each, so neither should the players.