I spent a long time running, playing and even writing for d20. It's a great system.
However, like most people who spent a long time with the system, I noticed how difficult high-level play was to manage. I believe this difficulty is what led to d20 being artificially capped at level 20.
Lately, I've been playing more and more AD&D and one thing that blows me away is how well that game handles high level play. d20 begins to spin out of control around level 15, while AD&D handles characters of 30th level with ease.
After giving this a lot of thought, here is what I believe to be the main culprit:
Never-ending Hit Dice.
In d20, a character gets a hit die every level. This vastly inflates hit points at high level.
In AD&D, you stop getting hit dice between 9th and 11th level and thereafter get a hit point bonus of between one and three hit points.
You also stop getting Constitution bonuses to hit points at the same time.
This means a 20th level AD&D character doesn't have many more hit points at all than his 10th level counterpart.
Whatever else you want to say about balance in d20, I think this fact alone causes most of its problems. Characters are harder to kill, combats take longer, and threats are much harder to scale up, requiring much more work to use adventures outside their suggested level range.