Everyone seems to be overthinking all this 'actions'-stuff. One action/round is self-explanatory, isn't it? I ready my weapon. I close in on the enemy. I cast a simple spell. I leap down from the balcony. I hit him with my sword

. I dodge the blow. I try to pick the lock. I push the table against the door.
I think the Omni-system (Talislanta, the original d20 system which only used a d20 for every action in the game) had one action-rounds.
Advantage of this type of approach is that it becomes so much simpler to teach the game, and it's so much more intuitive. Anyone, gamer or no, can grasp the concept, whereas move vs. standard is already counterintuitive. 2 move actions = 1 standard + 1 move action or 1 full-round action. But 2 standard actions is not equal to 2 move actions. Whereas they occur in exactly the same span of game time.
And once you start with free and swift and immediate actions, you're just complicating things unnecessarily. And, in the case of swift actions, basically to allow for the creation of a whole new slew of feats.
I also don't think that with one action/round, it would take forever for a melee character to get up close and personal. Especially with mini-free or light combat, the declaration of 'I close with the nearest enemy) should be enough. Unless the terrain is especially difficult or hazardous.
And there's also a balancing factor present for magic users vs. melee users: level of spell determines how many actions (= rounds) it takes to cast. A 9th-level spell becomes very hard to pull off in the midst of combat. But if you do make it, you probably win the battle.
Granted, it would make for a very different experience from the current gamist combat in both 4E and 3E/PF. But it would allow new gamers to pick up the basics far quicker, speed up actual play (no more analysis paralysis or painstaking combinations of several types of actions) and in that way improve the sense of immersion. Because in real life and in fiction, this actions budgeting never occurs.
(BTW, a game round should probably be approximately 3 seconds long in this system)