Honestly, out of all the complaints about Darius I've seen, the concern about his passive seems the most valid.
1 - Teemo's poison is his E, not his passive. Darius gets Hemo in addition to his QWER skill set.
2 - Teemo has to stack AP to increase its damage, meaning his basic attacks aren't very strong paired with it; if he builds AD, his poison damage is barely noticeable. Darius' passive scales off of AD (though it deals magic damage, which is a mixed blessing: it means stacking armor against him won't protect you from his passive's damage, but it also means the huge amount of armor penetration he has doesn't help his passive do more damage to you), meaning as he builds for mass damage his hemo gets beefed up to the point of ticking for hundreds of damage as well.
3 - Teemo's poison doesn't give him a speed boost when it's on someone. Darius gets +5% movespeed for every champ that's bleeding; this doesn't really help him escape, but it adds vital utility to an already surprisingly high-damage passive.
4 - Teemo's poison only activates on basic attacks. Darius' bleed stacks on you anytime he damages you with anything (note: his E (the pull) does no damage, so this doesn't add a bleed stack).
Point being, Teemo's poison is a pittance compared to Darius' passive.
Just for the sake of details, some numbers:
Magic Damage on Hit: 9 / 18 / 27 / 36 / 45 (+0.14 per ability power)
Magic Damage per Second: 6 / 12 / 18 / 24 / 30 (+0.14 per ability power)
So at max rank, shooting you once (UND ONLY VONCE) deals (45 +0.14AP) + ((30+0.14AP) x4) damage. So with an average Teemo build (Malady, Wriggle's Lantern, Frozen Mallet, Bloodrazor, Wit's End, let's say), his poison deals 48.5 + 33.5 x 4 (Total: 182.5) magic damage per shot. Not terrible for an auto-attack booster, right?
Darius' basic attacks and damaging abilities cause enemies to bleed for 12 / 15 / 18 / 21 / 24 / 27 / 30 / 33 / 36 (+0.4 per bonus attack damage) magic damage over 5 seconds, stacking up to 5 times.
Darius gains 5% movement speed for each bleeding enemy champion.
So at max rank, hitting you once does only 36 +0.4AD damage, and it takes 5 seconds to do that tiny amount of damage. If he hits you 5 times, however, it's doing 180 damage over those 5 seconds. Keep in mind, that's without ANY AD - it's already doing about as much damage as Teemo's E does with a pretty standard on-hit build completed. Let's say Darius actually has some items he'd want (average tanky dps build: Warmog's, Atma's Impaler, Bloodthirster...actually let's just stop there with three items). I'm not going to screw around with the health->damage conversion of Atma's so I'll just be conservative and say it's only giving him 44 AD, so he has 144 bonus AD.
Now if he hits you once, Hemo deals 93.6 magic damage over 5 seconds. If he hits you twice (which is pretty much guaranteed, since if he can hit you once he can hit you with any of his other abilities right away, whether it's Q->E->AA or AA->W or AA->Q or whatever), Hemo deals 187.2 over 5 seconds - already more than one shot of Teemo's poison. If he gets the full 5 stacks Hemo does 468 damage over 5 seconds. And this is with only two AD items in his build.
Yeah, I think concern about his passive is fair. Would I call it OP? No, but it's certainly damn strong.