I'm considering an E6 campaign, and I'd like it to have some long-term viability--which means I'd like to close the largest holes preemptively. (I really dislike ad hoc retcons in the middle of a campaign due to unanticipated problems.)
On the other hand, I don't want to shut things down if it's not necessary. One of the nice things about E6 is that you DON'T have to micromanage it, for the most part. I want players to be able to build the characters they want.
So, the question is: where does E6 break down? If you were trying to make an overpowered E6 character, what route would you take? Areas I've identified so far:
1. Artificer. This, to me, is the biggest hole. E6 strives to hit the "sweet spot" where casters and fighters are balanced, and, for the most part, it succeeds. Casters generally start to pull away at level 7 with the advent of fourth level spells. The Artificer, though, starts to pull away at 5, and spikes drastically at level 6. By level 6, the Artificer has basically all the features that make it better at everything than every other class.
The question is, can this be fixed without banning the Artificer? I've never made any secret of the fact that I'm a fan of the class, but I don't want it to be utterly dominant in the campaign setting.
2. Effigies. There's a world of potential for abuse here, particularly in conjunction with CL boosts (like the Artificer's!) It's easy to shut down the worst abuses by saying "No templates," but that still leaves people running around with 7-headed hydras, fully armored Annis grunts, and wyvern mounts...and that's with only a single CL boost and the Monster Manual!
3. Spellstitching. I'm fond of Necropolitans, but a Spellstitched Necropolitan can be trotting around with close to a dozen SLAs of up to 6th level. In a game where third level spells are king, that's...pretty fricking scary.
4. Dragonwrought kobolds. This one's not too hard a fix--just nix the epic feats and free sorcerer level tricks.