One thing that I would love to see changed and would probably have very little push back from anyone - fix the damn economy! The gold based economy is not only seriously unrealistic (not only in history, but also in most fiction), it's also dumb in a gaming sense. Copper and silver are pretty much useless. Also, players have little need for money to buy non-magical items.
In most other rpgs, you will start with the cheapest equipment and have to save up to afford things like plate armor. In D&D, you start off with good equipment and can afford the best equipment after 1 or 2 adventures. I think they have a lot of room to improve how this works.