Having been a hero dagger spec, I found my two main weaknesses were:

1. Tanking for indirect damage. I know it sounds crazy, but you'd be surprised how badly dodge/evade serves you against classes you can't lag. If I couldn't trip them, then invokers, bards, transmuters and shamans would really hurt. Taking them out was a matter of hitting and running because of that. They all have huge damage reduction instead of evasive defenses - that means you have no defenses to pierce, and find yourself very short on pure damage output. Dagger specs are also pretty bad versus a lot of mobs - although they probably out tank several other specs, they're not near the top with things like spears, swords and polearms. And these other specs can be easier to tweak-up in terms of melee damage (eg, polearms and trapping, swords+flurry, spears+felar). Even most of the better daggers don't have a very high average, and dagger specs won't gear as much for +dam.

2. Dex malediction. You skills rely on your dexterity, and having it nerfed by boneshatter/impale leaves you unable to tank AND use spec skills reliably. Because there is also a back-lash associated to dagger skills, this pretty much incapacitates you. If you want to go around teasing dagger specs, take maces. They're by no means a guaranteed win but they have all the tools required to hammer a dex-based warrior. Combine with whips for entwine, and a defensive legacy or two (riddle, gates, etc), and you've got a pure dagger-spec killer.

Hope that helps. The other solution is always to bring a friend

Yhorian