I don't think that the concern is that any given item is too powerful, but rather that certain items will "combine" in a way that is overly beneficial to a particular class/build. When I say "overly beneficial" what I mean is that a build using items with the proper synergistic abilities will be notably more powerful than that same build without. As splat books hit the market, and the number and variety of "optional" magic items expands, this will almost certainly be the case. Consider an "archer" build with items of the following type/ability. (I assume that these are times per day or times per encounter effects)
Boots: allow spiderclimb or levitation
Bracers: allow a ranged weapon to "autocrit"
Gloves: ignore some kind of miss chance
Belt: allow an extra move action
Headband: reduces range-to-hit penalties
Now some of that can be said to be situational, but there is no doubt that in a large number of common scenarios an archer with those items (in addition to the mathematically assumed weapon/armor/cloak) would perform much better than an archer without them. So while the assumptions may not be built into the math, there are (or rather, will be) combinations that are advantageous enough for given "builds" that PCs will feel the need to have them all, or a variation of them. We have heard from the developers that the "core" books assume the "big 3" magic items and nothing else. Now the question becomes that once "ideal" combinations of extra magic items come about, will supplements begin to assume that every archer has an uber boots-belt-glove combo, and design encounters with that as the new assumed standard?