I guess D&D 3.75e should have:
1. Clearly-defined categories of actions: immediate action (free, not your turn), swift action (free, your turn only), short action (what is now called "move-equivalent action"), standard action, and long action (what is now called "full-round action"). This section of the PHB should have the word "category" in it many, many times so that word will stick in peoples' minds when they read it. These are categories, not actions!
2. Categorized actions that don't contain any of these words: "immediate", "swift", "short", "standard", or "long". These are actions. You can take a long action, a short and a standard action, or 2 short actions during your turn. If desired, assign 0 points to swift/immediate actions, 2 points to short actions, 3 for standard actions, and 5 to long actions. You have 5 points to spend... use them or lose them!
3. Feat descriptions that clearly apply to either a specific action or a whole category of actions. A bit of fine feat balancing can be done at this point (Shot on the Run, for example, needs a little help IMO).
Limiting mechanisms for quickened spells will need to be implemented as well. Perhaps "quickened action" is a really small category of swift action that can only be performed once per round. Or it can simply be considered a special case of "swift action". Or all "swift actions" can only be done once per round, so the category already fits. Obviously, there's still a lot of rough spots.
The 5' step rules could be difficult too... I'd include it as an optional component in certain standard and long actions. There could be some strangeness like "I move 30' for my short action, then 5' step and attack for my standard action," but I think that would be bearable.