Basic and Leveling

From page 61 of Moldvay:

'If no one has reached the 2nd level of experience in three to four adventures, the DM should consider giving more treasure. If most of the players have reached the 3rd level of experience in this time, the DM should consider cutting down on the amount of treasure, or increasing the "toughness" of the monsters.'

I find this passage interesting for multiple reasons. 

1. It explicitly states a metric for leveling frequency (a topic that gets debating frequently on message boards like Dragonsfoot). Granted, it's talking about going from 1st to 2nd, but still. The implication is that somebody (probably the thief or cleric) should level in the first two adventures. Considering that Moldvay states earlier (p.19) that the typical party consists of 6-8 PCs (not including retainers), and a PC needs at least 1,200 to level (2,200 on average), that's approximately 4,000 experience in monsters and treasure being handed out per adventure.
2. The recommendation to adjust the "problem" is to hand out more loot. Implying that the majority of experience comes from treasure, not combat. This further reinforces the concept of the "exploration model" of adventuring. Your goal is not to fight, but to find. 
3. The term "toughness" in the final part of the above quote is somewhat vague. The concern here is that the party is having it all their way and the campaign needs to check itself before it wrecks itself. An encounter can be made harder in a number of ways, some of which would also increase the XP awards by the book. i.e. increasing the number appearing, but staying within the prescribed guidelines. There is another implication here, though. Encounters can be made tougher by upping hit points to maximum; adjusting –or even forgoing– morale checks and reaction rolls (assuming hostility and fighting to the last); or even using more dangerous creatures. The idea being that fewer PCs survive to level at all! That's one way to solve the problem (perceived or otherwise).