One of the biggest problems I encountered in 3rd edition was the ability for characters to have not just one, but several pets, summoned monsters, animated dead, rebuked undead, crafted golems, charmed/dominated creatures, followers, etc.
The Summon Monster/Nature's Ally spells allowed you to summon 1d4 + 1 monsters with a single casting, and each one was its own creature with its own entire turn, attacks (often several attacks), spells, etc. And there was no restriction against casting the spell multiple times to gain more and more monsters under your command. Sorcerers and Druids in particular had a very easy time abusing this.
The Animate Dead spell was also problematic. While there was a limit on how many creatures you could control at a time, the limit was 4 times your caster level in HD. A 10th level caster, for example, could have 40 HD of undead animated and serving him at any given time. That could be four hill giant skeletons, for example! Clerics could take it even furhter, by adding their level in HD of commanded undead.
Summoned and animated dead were in addition to any golems the character crafted, animal companions, familiars, henchmen, outsiders from planar ally/binding spells and beings the character had dominated (which lasted for days, even weeks). Between all of these things, and others, characters could have a ridiculous number of creatures under their command. 4e had its action economy to solve this problem, but I think it may have gone a bit too far in the other direction (even rangers had to share actions with their animal companions, for example).
I hope something is done to reign this in in Next, without having to go to 4e's extreme of shared actions. For example, I think that summon monster spells should usually be limited to summoning a single monster and should require concentration, preventing you from stacking them. Dominate Person/Monster should also require concentration, preventing people from having dominated monsters + summoned monsters at the same time. Animate Dead should be limited to creating one minion at a time. These are all sensible restrictions, IMO, and would at least keep things relatively sane.