Which monster do you use, and when? How much treasure do you give out? Matching up foes with PCs of a given level, as well as giving guidance to Dungeon Masters regarding how much treasure to award PCs of a given level, have been a hallmark of the game for the last fifteen years.
Sometimes I'm actually surprised at the comments coming from the designers for this "hallelujah" edition that will unite all others. When I see their comments it's almost as if their only experience with the game started about 10 years ago.
I know that Gary's DMG was a masterpiece of disorganization. It's hard to look at that and find stuff in it easily. However, he does give guidance for monster placement, as well as treasure placement. He even goes into great detail explaining about the DM being careful with the placement of magic items as they can unbalance and break the campaign.
The "level" system for monster placement was so explicit as to make it almost moron proof. Dungeons had levels, and it was expected that monsters would be placed in a dungeon using that level as a guide. The deeper in the dungeon the higher the level, and the more challenge that should be expected. How much easier can it get? Each monster also had a treasure type that could be used by the DM to determine what type of treasure the creature had. He could roll randomly for the treasure or pick from the multitude of tables.
The level system also made it so that the players could decide the level of challenge they were willing to endure. A first level party knew that going to the 8th level of the dungeon was probably suicide, but the payoff was great too.
The XP system even mentioned to gauge XP rewards based on level of difficulty. Gold was turned to XP and if the challenge was low the XP reward was not 1 to 1. Not only that the creatures treasure was usually "thematically" appropriate for the creature. Something the Wealth by Level Guidelines never really addressed.
I know that the CR system and the Wealth by Level system existed starting with 3.x but I'd be hard pressed to call the CR system a hallmark of design. As a matter of fact placing monsters using 1e guidelines is much more accurate than anything I ever experienced using the CR system. My overall experience ended up with about a 70% inaccurate ratings for the CR system.
Don't get me wrong. I'm not looking at 1e with rose colored glasses and saying that finding that information was easy. I admit that had it been formatted in a much more "precise" manner it would have been much better.
I just don't see the CR system and the Wealth by Level Guidelines as an improvement of design. They were better organized, I'll give them that.