Yeah Humble, why no teemo love? I expect him to do well.
Already explained it above somewhat. Here is a bit more detail as to why. I'll break down Teemo's problem as either an attacker role or defender role.
Teemo as an attacker. Attackers need good mobility and dps.
W is great for mobility. He can get around the map fast enough to help take or show up for a added defense on a tower. Not the fastest, but still good.
DPS is sadly where teemo is going to fail here. He is regulated to auto attacks and toxic darts. Blind isnt all that useful against most of the defender style chars I put on that list. Try and blind a heimer dinger defending a tower and he's going to laugh at teemo while his turrets beat him down. Any dps he may have from mushrooms as and attacker is going to be negligible. Defenders are going to see you lay one down near a tower and they aren't going to be stupid enough to leave their defensive position to step on your shroom.
So while teemo does have good mobility to be an attacker, he lacks on DPS to take down a well entrenched defender. Especially on his own.
Teemo as a defender. Defenders need good sustain, spammable damage, and HARD CC.
Teemo is better as a defender than an attacker. While he lacks spammable abilities, dot damage makes up for that somewhat.
As a defender his W isn't nearly as important so that leaves that out.
Blind is somewhat useful, but most attackers rush in to get defenders to waste their CC abilities, and either cleanse them off or back off a second before going back in to finish the job. So while it's decent, it's not great. CC is useful to a defender, but overall CC isn't nearly as useful in this game. CC is more useful in Classic because typically people are suppose to dogpile and gank on a CC'd target. In dominion most players are alone at most with 1 other player. CC isn't as useful when you aren't able to capitalize and kill someone once your CC goes off. Also as a defender you need hard CC, not soft CC. As a defender you are going to be sitting on a tower that will hopefully dish out quite a bit of damage to any attackers that come close enough. To capitalize upon that damage from the tower you need at least a massive slow; but a root or stun is more preferable. A blind or silence is not really as useful.
AoE is useful for a defender and Teemo somewhat lacks that. With one exception: His mushrooms. These are actually sort of good to use as a defender. The problem is the long setup time it takes to place any decent amount around an area. Unlike Shaco or Heimer or other champs that spawn a "defender" ability, shrooms don't do much until someone walks over them. If someone see you lay down a shroom then it's wasted as that player is not going to activate it. If someone comes to attack the tower that Teemo is defending through a route which doesn't have shrooms placed then those shrooms placed else where are negated as well.
As far as sustain goes... Teemo is one of the squishier champs. He has no built in shield, health regen, life steal, or anything else that would make being a defender needed. Sure he can use items to make him better, but so can the attackers get items to overwhelm that.
So on a whole teemo makes a crappy attacker and only at best a mediocre defender. He lacks the needed DPS to take down defenders rapidly as an attacker. As a defender he lacks sustain and his shrooms as defense are dubious at best due to set up time to place them, and an attackers ability to avoid them all together. He also lacks spammable abilities for interrupting, but at least his dot damage sort of helps back out with that. He has no hard CC or reliable AoE either for use in defense. There are much better champs for either an attacker or defender than Teemo.