For single-classed characters, 8th level is a nice spot; but I like groups of multi-classed characters which become really fun in the mid-teens with two base classes of 4 or so levels plus a prestige class or continuing one of the base classes. While it does slightly delay the power curve, the main effect is to produce more rounded and more balanced characters. Combat types tend to have decent saves, mages have more HP and better saves, characters have interesting abilities (e.g. Barbarian + Rogue), aren't so focussed and don't have those glass jaws. It doesn't matter that the spellcasters are one or two spell levels behind. If they can only cast Dispel Magic, I'm not going to put them in an adventure that requires Teleport, even if they're 15th level (e.g. Mk 4 / Pal 4 / Sor 7). Despite being 21st level, I wouldn't pit a Ftr 10 / Wiz 11 against Kyuss; Ftr 10 / Wiz 15 maybe.
As for the plethora of magic items, I find a good churn does wonders, and Disjunction's always good for that. And if someone's prized possession gets Disjoined, well, that's a good adventure hook.
But really, you can make an argument for any level.