And that's fair, to a degree, I feel like. But, why?
It's not that the areas aren't varied-they are.
It's not that there aren't some threats-Rares helped there.
I've heard people say that it's due to the similarity of floors within a dungeon. But that's not really dull, is it? They're only three floors tall, after all.
I think of the areas I find more interesting, and they're places like Heart of the Gloom-places that mutate a bit depending on the level you go in. Maybe that's what's lacking-that there's similar situations all the time, and that the opponents don't do interesting things.
But at the same time, you can't make your opponents too strong. Even Ritches have fairly simple skillsets-yet that dungeon is incredibly dangerous.
Of course, skills don't have to show up at the base level of the enemy, do they? People like to do multiple starter dungeons...so maybe enemies past L5 on the critter level should get some skills.
And not all the critters or earlier enemies are bad, either. Bears and Trolls are, for instance, semi-competent and can cause you to have to wake up occasionally. Aquatic critters are, outside of Electric Eels being nasty due to having such high rates of fire, quite interesting without being particularly powerful. So, this obviously can be done.
Here's some thoughts I've been having along these lines:
Rodents: The rodent dream gives me a thought here: Rogue and Mobility type skills fit reasonable for rats. Giant Mice
could get L1 Stealth and Shadowstrike at their level cap(which is quite low)-they don't do a lot of damage, but ignoring them can be at least marginally risky, this way.
Giant Rats could be the more aggressive type, getting Nimble Movements and Lacerating Strikes at their level cap, and maybe marginally better damage and durability-this combination would allow them to get on top of you fast and threaten low save setups with a little bit of damage, but again, only really threatening to someone in tandem with another enemy, and only at higher levels.
Neither of these are likely to do anything but make them stand out a little more while you're splattering them-which is, of course, the point.
Giant Rabbits are inherently silly. Maybe they should just summon other rabbits.
Giant Crystal Rats, the unusual lord of the rats, could be Corruption themed and get(at their level cap) something like Acid Blood and an extremely low magic Virulent Disease, or could be Arcane themed and get Stone Skin and an extremely low magic Strike, or even could just be regular, but unusual, nature infused rats that could, say, Burrow and Nature's Touch at their level cap. (Or maybe not Burrow-after all, rat nests exist...)
Canines: Wolves have no level cap, unlike rodents. They also appear a bit later.
Standard Wolves could Rush you at L10/gain a level in it every ten levels, while the Greater and Dire Wolves could use Knockback and Crush respectively. This, at least, would make them as interesting as Trolls, if less competent overall.
Wargs are meant to be smarter wolves: Perhaps getting an infusion slot would make sense for them? Many natural enemies seem to have infusions.
Foxes should probably have Rushing Claws at L10/every ten.
White Wolves should probably get Icy Claw at L10/every 15 for later levels, Icy Skin at L15/every 20, and Ice Breath at L20/every 20 for later levels. This makes them stand out, and fits them quite well.
This would make White Wolves stand out as the notable "king" of wolves, while the rest are about fast strikes. A little boring, but more interesting than they currently are, without strongly powering them.
Ants:
White and Brown Ants could stand to be generic. Carpenter Ants...I would suggest Burrow but I have horrible visions of ant filled pits spilling out into the landscape, so they can be reasonably generic too.
Green ants could pick up Spit Poison at L10/every 20. Red Ants, the Ritch Firespit at the same level. Blue, Water Bolt, and Yellow, Call Lightning. Black Ants, Spit Acid.
The higher tier ants only show up at fairly high levels(L15 and up), and could stand to be spicier. Fire Ants could get the special Ritch blastwave skill(Flame Fury?) from the start, as well as Firespit, both at their base level, with extra levels every 10. Ice Ants, Water Jet and Icy Skin. Lightning Ants, Call Lightning and Lightning Speed. Acid Ants, Acidic Spray and Spit Acid.
They have extremely low statistics, so these would only be capable of doing 20/30 damage, I'd imagine. Not too bad, in a hallway, though it makes ant nests at least vaguely threatening, which is more interesting.
Army Ants, the highest level ant at L18 and up, could have Stun and Disarm for their skillset. L1 to start, with more levels every ten after their base of 18.
(Baby) Dragons:
The baby dragons are boring sacks of HP for the entire game. Which makes them fairly competent really early, so that's no problem.
Spicing them up later is easy: Wait until L35 and up, L40 to be safe(when Wyrms are generally managable quite easily), then have the fire ones gain Devouring Flame L1, the Air ones get Lightning Speed L1, and the Acid ones gain Acidic Spray L1.
They're fairly bad with these, so it should be okay, usually-yet makes them a credible support for the big dragon, and generally makes things more interesting.
Multihued hatchlings are a little more difficult, but they're also sorta nearly competent at all levels, so maybe that's okay. Besides, they have the more dangerous parents.
Jelly:
Jelly, along with Mold, are the saddest things. They can't move, they can't summon more of themselves, they just have to sit there and die to anything with range over 2...and they don't threaten at range 1. Poor things. At least the Angband versions could break your potions annoyingly.
I suggest giving them all Slime Spit L1 at L5/every ten levels after-this makes them turrets, on base, albiet bad ones.
For White Jellies, this is enough. For the rest, however, a simple trick makes them more interesting: They still have the same Slime Spit White Jellies have, but have 100% damage conversion on it to their respective element. So they spit, say, lightning slime for a Yellow Jelly. Simple!
Also, I'm avoiding the critter uniques here, as that's really another topic... but with a name like Malevolent Dimensional Jelly, you'd figure it'd be more competent.

Mold:
Probably the saddest things.

My thought here is simple: Give them a little more HP, firstly. Secondly, give them Multiply at all levels. And thirdly, give them Slime Roots at L10/every ten after.
Now they at least can gum up the place with more of themselves and wander around after you vaguely at higher levels. Kinda like less competent, less annoying Oozes, which, given that oozes tend to show up later, and with better stats, works fine.
Plants:
Giant Venus Flytrap: Constrict(at all levels) and Spit Poison(past L5/every ten). Fits okay.
Poison Ivy: Poisonous Spores past L10/every ten. It's tempting to pass this skill out to a lot of enemies, but it's very good so only the ones that are really iffy should have it. Like these guys.
Snakes:
King Cobras should Spit Poison, Black Mambas should Constrict at a lower level than Anacondas, and Anacondas should Crush and Stun their prey. Snakes are otherwise one of the more competent enemies relative to their status, so they don't need a ton of help.
Swarms:
Die so easily.
The first two should have the Evasion skill, so they can take melee(L5/every 10) but not ranged attacks. Hornet Swarms and Hummerhorns should have that too, but also get Heightened Reflexes(L10/every 10), so that nailing them with a projectile is harder. This makes them annoying, which is how a swarm of bugs should be.

So yeah.
These are just some thoughts. I don't want to see the earlygame get too hard-ToME has a very nice difficulty curve to it, and it allows even some fairly unorthodox builds enough time to get off the ground thanks to that. But some abilities to spice up the really shoddy enemies would make things more interesting, is my thought.
So, any thoughts?
Edit: Now mostly in add-on form! Some ideas were changed, most notably hatchling testing didn't...go well, let's put it that way. (Gorbat has way too many to give them any ranged attacks.)
http://te4.org/games/addons/tome/critte ... ter-tweaks Feel free to try it out!