Actually, I feel stuff breaks at different levels depending on what part of 3.5 we talk about. But it is somewhere above level 4 and below level 10.
There's really strong spells at all spell levels, but the comparative power level of 4th to 3rd is noticeable. Mostly that's Polymorph's fault.
There's the old issue with Skills breaking down by level 9, or more accurately having broken down by 9th level. I used to have a quote on this one, but it was basically something like a bard attracting planar attention with their music and other examples, I can recall Balance, Survival and Swimming, but only vaguely. The thing referred here is the DC 30 perform check examples given in the book.
Let's see, 9th level, 12 ranks. Charisma mod of +6, assuming 18 base stat with two stat ups at 4th and 8th alongside a +2 Cha magic item. Masterwork the item for the +2 circumstance bonus. That's a +20 modifier to your Perform check, at level 9. And that isn't even trying to break things, I feel most new players who would sit down with the Core Only books and who wants to be good at performing will make all these choices, or close to them. They're intuitive.
Heck, there's the old Binder workhorse item in the DMG as well, Circlet of Persuasion for another +3 competence bonus.
Now I have less problems with the skill system if you confine it to E6 or E8. Because the implicit assumption is that by 6/8th level, you are Epic in that word by comparison. Then I don't mind the idea that a world-renown bard, the veritable Elvis or Edith Piaf or Michael Jackson or Madonna, is level 6 or 8.
But in regular DnD, if you are hitting the highest normal printed DC of a perform check on average dice by level 9, what the game tells me is that you manage what is deemed "very difficult" trivially by the point you are halfway into your career.
Other issues is that the verisimilitude and world building have potential pitfalls. I know they didn't bother with a economical system that works because it's only supposed to interact with the players on a small scale. But as a world builder and the DM, I want a world that makes sense to me, given what is in it. Going by the DMG and looking at how many NPCss there should be in various sized communities, I can't help but notice that Plant Growth really should have an impact on the way farming work. Gah. Simply put, I feel the baseline assumption in the DMG about how many spellcasters are around ineveitably would impact how the world building needs to go. I have a hard time putting this into words, though.
