I know that for many encounter tables are not seen as a problem. Their campaigns are a carefully designed parade of confrontations with prepared monster groups, each stage of combat culminating in an inevitable treasure-providing climax. But for those of us for whom the game is a simulation, encounter tables are an enormous headache. Because they are, in a word, shit.
I wrote about that eleven months ago, so I can skip it. Instead I want to focus on just a few problems with specific monsters, and suggestions how they might get used.
There are two groups of creatures I have found consistently annoying. The first comprises the collection of insidious grub-like killers: the ear seeker, the rot grub and the throat leech, to name three prime examples.
You have to be some kind of sadist as a DM to use these things. I presume their primary function is to wear the party down, cost them a few hit points or deplete their spell supply. Seriously, I've always had trouble using them. Now and then I've had a pile of rot grub kill the expendable NPC, but I have never actually had a party member drink a throat leech or have an ear seeker invade a sleeping character. The idea that an ear seeker would happen to be in the exact place where a player puts his ear to a door is ridiculous. The only place it occurs on the original DMG encounter tables is the Level 1 Dungeon Table ... where it has a 1 in 100 chance of being present. What happens if no one happens to lay down or press their ear to something in that room?
What it actually means is that you must reroll ... which is what we all do when these things are indicated. We reroll, and reroll, and reroll, until we get something practical like orcs.
In all honesty, the only practical use for these monsters is as a trap - the table on which these things should appear. The only way that an ear seeker is going to get shoved in your ear is if somehow there's a device that puts it there. In other words, at the bottom of the pit there are spikes, or someone has carefully collected four hundred rot grub. The same might be applied to a pool, when entering it and failing to save causes you to sleep and allows your mouth to open - at once, one of thousands of leeches immediately enters your gullet. Now, not only does the party have to save you from drowning, they must heat up some kind of wire (after pulling themselves from the wet) to keep you from choking. Nasty.
Alternately, how about a creature which specifically lives in tandem with ear-seekers or throat leeches? That is, it strikes you in the side of your head, and on a natural 20 it launches an ear seeker straight into your ear. That would make you think.
Otherwise you're creating scenarios where the ear seeker tactically drops from the ceiling, hits you dead on in the ear and scurries in before you can say boo. Not bad aim for a non-intelligent creature. It must have a sensory device that detects ears moving underneath and provides spot bombing ability.
I actually had a DM do this in a game once. The encounter table said ear seeker and damned if we didn't get one with Olympic-level vaulting capabilities.
The other creature group that I despise comprises of plants which either a) move very, very slowly, or b) not at all. There are a bunch in the Fiend Folio, but let me concentrate on the shrieker and the violet fungi, as they are older and representative to the genre.
The shrieker we all know well. Low AC, lots of hit points and hit dice, good experience for low level parties. If you're lucky, you can kill it before the "50% chance of attracting wandering monsters" kicks in.
Seriously, if there was such a creature, terrifically safe to have around (no attack, no damage), wouldn't every humanoid on the planet cultivate these things in every hallway as a natural alarm system? Assuming of course that every rat, centipede or beetle didn't make it shriek fifteen times a day, pretty much eradicating anyone's interest in coming to check out what's making the fucking thing go off this time. The creature's very presence in the monster manual suggests that somehow this dungeon must be completely unviolated by the movement of any creature except the party - effectively existing in suspended animation. Sort of a Descartian universe: nothing exists until it is detected by the party's five senses.
Again, presuppose a trap. The shrieker sits in an enclosed, dark space, completely inviolate; you feed it from time to time. The party enters a hallway, trips the trap and the shrieker drops out of its box, landing on a party member. It shrieks. Guards come a'running in force.
Alternately, imagine the following signal system: build a tight, lightless box, with a small sliding door, just two inches high, that allows in light; open the slide, the shrieker screams. Close it and the shrieker calms down and stops screaming. Provide one of these boxes at every guard entrance - alarm problem solved.
The violet fungi is another problem. The largest is described as being 7' tall and having 4' branches. Assuming the body is two feet wide, this gives it a 10' reach. Mystically, this creature is described as having a movement of 1" ... in strict AD&D terms, in the outdoors this is 10 yards per one-minute round, or six-inches a second. That is a fast-moving plant. (The shrieker moves at the same pace)
Still, its only 1/6th as fast as a fighter in plate mail, so unless you are the blunderer for all ages, this thing isn't going to touch you. I suppose it is assumed that either you aren't going to notice it (oh look, another seven-foot-tall purple mushroom - I must get one of those for the den) or its going to be in the way just when you really need to go down that hallway.
It's pretty stupid to assume this thing isn't just going to get killed by a lit bottle of oil or a barrage of arrows; so again, we're not talking about something that's an actual threat, just something to drain off the party's resources.
Which brings us back to the trap. Put four of these things at the bottom of a ten-foot pit. Screw the crocodiles - when the party is working its way along the ledge, have the huge floor below overgrown with violet fungi. Guaranteed to kill even the hardiest 20th level characters. Let's see, 2' diameter, approximately 3.14 square feet per fungi, allowing for 50% empty space between creatures, equals 21.23142 violet fungi per 10'x10' square, multiplied by an average of 2.5 branches allows 53 potential attacks per round - well, less really, as the fungi themselves get in the way, lets cut it in half and say 27 ... 3HD creatures have a THACO of 16, 14 from behind, the shield protecting against only 17% of the attacks ... could hurt.
Those are just some recent thoughts, put here for interests sake.