I wrote this on another forum about the same question:
1. Magic is brokenly powerful. At level 20, characters based around hitting things are suddenly much much weaker than their reality-altering friends.
2. Polymorph effects are brokenly powerful. Shapechange and Polymorph Any Object are the worst offenders, but even Alter Self is VERY strong.
3. Magic causes problems. Related to #1, but besides sheer power, stuff like Scry-and-Fry (scry on target, teleport party on hapless victim, ambush, kill, teleport home) and Rope Trick (use up all your per-day resources in one or two fights, cast Rope Trick, safely take an 8 hour nap and reload) can really mess up a campaign.
4. Fighters sucked at higher levels sad.gif.
5. Combat at high levels take a lot of real time to resolve, but don't tend to last very long in-game (no more than 3 turns usually).
6. #5 means that any effect that removes a character from combat, like a Fear effect, means one player will sit there for 2 hours doing nothing.
7. At high levels, the sheer amount of modifiers make it very difficult to keep track of every little bonus and penalty. Even at lower levels, say the Fighter gets hit by a spell that reduces his strength. Since so many things are tied to ability scores, he'd have to recalculate a ton of different modifiers. This takes a long time and is prone to error.
8. At higher levels, gear is integral to a character's power level. Losing a +10 weapon is much much worse than death. It's not something you can shrug off either; a level 20 Fighter with a +1 weapon is perhaps on the same power level as a level 16 Fighter with a +10 weapon, all other gear equal.
9. NPCs require a great deal of time to stat out, putting a lot of workload on the DM.
10. Encounters are difficult to design. If you want to add a bunch of weak minions, for example, the encounter design guidelines means those minions will usually either be too tough defensively or too weak offensively. Putting in a single monster against the party is also a bad idea, since the party will have much more actions than the monster and spell effects can easily render that single monster very harmless.
1. Magic is brokenly powerful. At level 20, characters based around hitting things are suddenly much much weaker than their reality-altering friends.
2. Polymorph effects are brokenly powerful. Shapechange and Polymorph Any Object are the worst offenders, but even Alter Self is VERY strong.
3. Magic causes problems. Related to #1, but besides sheer power, stuff like Scry-and-Fry (scry on target, teleport party on hapless victim, ambush, kill, teleport home) and Rope Trick (use up all your per-day resources in one or two fights, cast Rope Trick, safely take an 8 hour nap and reload) can really mess up a campaign.
4. Fighters sucked at higher levels sad.gif.
5. Combat at high levels take a lot of real time to resolve, but don't tend to last very long in-game (no more than 3 turns usually).
6. #5 means that any effect that removes a character from combat, like a Fear effect, means one player will sit there for 2 hours doing nothing.
7. At high levels, the sheer amount of modifiers make it very difficult to keep track of every little bonus and penalty. Even at lower levels, say the Fighter gets hit by a spell that reduces his strength. Since so many things are tied to ability scores, he'd have to recalculate a ton of different modifiers. This takes a long time and is prone to error.
8. At higher levels, gear is integral to a character's power level. Losing a +10 weapon is much much worse than death. It's not something you can shrug off either; a level 20 Fighter with a +1 weapon is perhaps on the same power level as a level 16 Fighter with a +10 weapon, all other gear equal.
9. NPCs require a great deal of time to stat out, putting a lot of workload on the DM.
10. Encounters are difficult to design. If you want to add a bunch of weak minions, for example, the encounter design guidelines means those minions will usually either be too tough defensively or too weak offensively. Putting in a single monster against the party is also a bad idea, since the party will have much more actions than the monster and spell effects can easily render that single monster very harmless.