After beginning the thread, I almost immediately noticed two things: First, the book is an absolute treasure-trove of plot hooks for adventures and campaigns. I would estimate that at least 90% of the monster entries have something in them that an average DM will read and instantly have fireworks of inspiration exploding in his brain. There is so much good stuff in there that it almost feels like cheating to own the thing; it becomes criminally easy to come up with brilliant adventure hooks. In a low-key way, I feel like The Beatles when they discovered LSD and came up with Sergeant Pepper's Lonely Heart's Club Band.
Second, I realised that I haven't been anywhere near as creative enough as a DM in my choices of monsters. For a long time I've been trying to get away from the regular pathway of kobolds > goblins > orcs > gnolls which adventurers tend to follow as they go up in levels and power, but I've mainly brought this about by making player character races, usually humans and elves, the chief antagonists in my games. I think generally speaking it has been well-received because of its novelty value by my players, but looking through the Monstrous Manual has brought home to me just how much I've been missing out on: the best alternative to trope monsters isn't to put a new twist on PC races; it's other monsters! This post in particular opened my eyes to the possibilities of radically different low-level D&D campaigns:
The way I see it [talking about the idea of a campaign incorporating stag beetles and ankhegs], the party is hired by a powerful merchant consortium. A huge swarm of giant stag beetles, their eggs transported to the food belts owned by these merchants either by malice or bad luck, are eating all the grain in sight. Without that grain, not only is there no bread for the peasants and princes alike, but the barley that would go to making beer for the dwarves, an important trade commodity with those surly folk. With no beer, the trade in fine dwarf-craft weapons would trickle out, and the dwarves might spitefully look the other way if raiding parties of hobgoblins or kobolds were to move through the outskirts of their territory to raid the merchants' lands...Absolute gold, and something I just wouldn't have come up myself. With nary an orc in sight, either. (Okay, it has hobgoblins and kobolds in, but those are easily replaced by, say, the tribe of Broken Ones who live just beyond the dwarves' lands.)
So where do the adventurers come into this? After a few jobs just clearing out areas where the beetles are thick, the merchants realize that a more subtle approach needs to be taken to get all of the chittering menaces. Biological control. The party is then hired to delve into ankheg nests and steal enough eggs to introduce throughout the region; ankhegs are notorious predators of giant stag beetles, after all. But ankhegs in the region are nowhere to be seen. What sort of monster could be keeping the ankheg populations so small and eat their eggs...?
The thread has now taken on the tone of a kind of extended brainstorming session, where certain regular contributers can be relied upon to come up with the goods on an entry-by-entry basis. I don't feel guilty about yoinking any of their ideas, and as I happen to be in the middle of planning a Play-by-Post Rules Cyclopedia game to 'celebrate' (if that is the right word) the release of 4e, I can guarantee a large portion of the ideas from that thread will end up being incorporated. One of these is the riff on Giant Crayfish on page 36, part of which went:
[Me]: Giant Crayfish continue growing as they get older - which gives me an idea for a Crayfish which has lived in the same lake for millenia and is now the size of a blue whale. The people living around could have all sorts of legends about the beast, giving it the name Old Johnny or Big Bertha or something suiting.In a strange way this coincides with an interesting little discussion in the comments section of the grognardia blog, regarding OD&D's conceptualisation of religion (or lack of it). As James, the blog's author, notes, in OD&D there really were no gods, at least originally - with a host of "demigods, godlings, saints, demon lords, and high devils" existing instead. The Crayfish-demigod-in-the-lake idea is very much in line with that tradition, and it has inspired me to use the "no real gods, just spirits and godlings" concept as the basis of religion in my Rules Cyclopedia game.
[kelvingreen]: The backward villagers living on the shore of the lake worship the giant lobster as a god, sacrificing travellers to it every so often.
[Milk]: They've worshipped it for so long that it's become a god, albeit a small one. This demigod knows only its own cold hunger, and inspires that hunger in its followers. The villagers intentionally spread rumours to the outside world of a legendary giant crayfish that dwells in the lake, ripe for the catching, and watch as would-be fishermen go out onto the lake never to return.