Ahh... Mister Brain just got fully fired up. Let's see... nasty, nasty stuff. Well, here are some annoying ideas to throw at people.
Holly maze - or variant thereof, such as walls coated with shards of broken glass. And floors. Sensible clothing will avert damage, until something pushes you into a wall - minotaur gang bullrushing (ouch),
grease, or even just acid melting your boots. It's simple and cruel, just what a kobold would go for, and if they have their own tunnels they don't have to use these ones anyway.
Ball bearings. At first, ball bearings are annoying when they fall on you out of the ceiling. Then they begin falling harder, and in greater numbers. Pretty soon, you're scrabbling for your footing (like, say, Home Alone) and trying to avoid not only death from above but any number of pit traps, acid baths, etc. It's like
grease but, well, made of metal. (If you don't have ball bearings, use quicksilver or small lead balls, smaller than sling bullets.)
The old staircase => ramp trick. A steep marble staircase whose second-top step triggers the whole set to rotate, turning it into a slippery slope. Bonus points for having a wall with big spikes at the bottom; the PCs are bound to wonder about that when they start climbing the stairs...
Catapults. Bouncy is always fun. A catapult can hurl someone high into the air and then down again, preferably onto something sharp and/or likely to cause skin irritation. You could easily use this in the bottom of a pit trap. And hey, a low ceiling is no penalty to a catapult; you go splat for the full distance anyway. EDIT: You need Profesion: Siege Engineer to properly aim catapults, right? Kobolds wouldn't be too swell at aiming right, which can cause for some truly nasty scrapes (catapulted down the hall, off the wall, and over the pit of spikes, where you bounce off the wall again and back into the spikes).
Slide Of Doom: Oh, look. A chute. It's just the right height for any small humanoid to slide down safely. Unfortunately, around the first corner, the blades come out of the walls at arm height on Medium-size creatures. Whee! The kobolds are fine; the big invaders are suddenly armless. And harmless. Bwaha. Bonus points if you land in vinegar or salt at the bottom. The screams would be heartwarming.
In fact, send kobolds out with bags of salt to throw on their enemies after every injurious encounter. I'd require a moderate Fortitude save to avoid some sort of pain penalty (perhaps the nausea condition would do the trick).
The next two traps are highly vicious and may cause offense; you are warned. The kobolds would think them to be hilarious, however.
The Nutcracker. One of those nasty things you invent when you're young and full of pep. A corridor, just wide enough for a human to walk along. Halfway down, they trigger the trigger, and discover that the floor is divided into three sections across the passage; the outermost two are hinged off the central one. The outer sections fall; the inner one doesn't. Those males who fail their saves fall roughly three feet and find themselves in mind-numbing pain as they straddle the central support. (Then, of course, something attacks and half the party is nauseated, or the effect of choice to reflect their 'injury'; anyone who failed their save is effectively prone, straddling the central barrier, and if they regain their feet they have to make Balance checks to fight on the central barrier.) Kobolds just stick to the inner track.
The Nutslicer: I just thought of this one, and it's mean. There is no central support in this variant; the floor is metal, and hinges down to a knife edge. This one does damage, probably more damage for every encumburance rating a PC has; say 1d6 for someone with light encumburance and no armour, with +1d6 for every unit of encumburance or armour (up to 6d6 for someone with full plate and a full loot sack). Of course, male PCs suffer the same nauseation effect, and will DEFINTELY need medical attention afterwards.
I may remember more from my glorious youth... you have been warned.