Characters had a wealth by level guideline that was very high (and indeed unsustainable, based on typical amounts of treasure gained per encounter).
The system assumed you were covered in magic items. If you weren't, certain stats (AC and saves, plus your main ability score - critical for spellcasters) fell so low the inbalance with other stats was very pronounced and obvious, and monsters would have you for lunch. (Contrast with attack bonuses, which would still increase.)
DMs could have trouble just assigning treasure. I used a random treasure generator and not only were the amounts it was giving out mind-boggling, but it turned out to not be enough. Too much scrambling to a big city like Sharn in order to sell loot for decent gear. It felt like Diablo, where unless it's a boss fight, you aren't running home due to lack of hit points or mana, but because your treasure box is full. At one point (I was a PC in this Eberron campaign) we had filled all our bags of holding and were reduced to only looting enemy rings (essentially weightless, and usually high value too).
The items were so integrated within the rules that most attempts to create low-magic item systems in 3.x were doomed to failure.
Contrast to 4e, where many items (those that boost stats) don't exist (except in name), where character defenses actually improve with levels rather than items, and the only items you have to boost are weapons or implements, armor (or cloth) and cloaks of resistance. At a mere three, you can create low-magic-item rules that actually make sense. (The inherent bonus system effectively replaces those three items.)
The management system is also easier. In theory magic items have cost, but that has nothing to do with balance. (I think the same thing of rarity; not only do I not know what this concept even is, I don't care and don't intend to learn.) Instead I can just use level.