The new player experience, as regards making the first character, as presented in the core rules on PHB p. 4-5. I'm talking about complete newbie material, a newcomer being invited to RPGs for the very first time.
The number of decisions to make at 1st level should be very small.
D&D should be easy to get into, easy to make the first character, like in OD&D: pick race, class, equipment, and you're done. Character customization is a good thing (skills, feats, more spells, domains, etc.) but the problem is it's front-loaded at 1st level for the new player.
Ideally, it should be the other way around: very few choices to be made at 1st level; more and more options opening up, as rules-mastery grows with the higher-level character. If you want a wide-open campaign for expert players, have it start at a higher level. But the way it stands now, the greatest number of decisions (ability placement, skill points to spend, feats to pick, spells to select, etc.) all occur at 1st level, where it is a barrier-to-entry to the complete newbie trying to enter the system.
That's the #1 thing I'd like to see fixed in the d20 System version of D&D.