Traps are one thing. If your kobold lair doesn't have traps, you're doing it wrong.I seem to recall kobolds being really good with traps though, so maybe not completely outside the realm of possibility. Maybe Goblin Slayer would be another example? It is the monster's home that the PCs are invading, they should have some plan.
But yeah, smart and proactive monsters can fuck your PCs up. I'd rather have those than punching bags or monsters that behave like WoW mobs.
I'm talking about (as Roger E. Moore) shit like 'kobolds firing from murder holes in the walls and ceiling, plus kobolds in metal armor pushing flaming barricades while other kobolds flung molotov cocktails'.
As one letter writer to Dragon later pointed out, stop using kobolds for this shit. Use classed NPCs, or some of the smarter monsters (hobgoblins, despite having a rep for hating magic, are -really- good at alchemy and engineering in the fluff). There's no need to subscribe some of these weaker critters to a tactical correspondence course.