I like the standard, move, minor (SMM) system for the most part. I have been experimenting with removing the move action and making the action system one of simultaneous actions where the Standard and Minor actions represent the degree of attention required to pull something off (as opposed to time, for example). Movement, talking, etc. are simultaneous things that normally aren't "actions" at all, just things to do at any time during the turn. Some things that are actions (e.g. melee attacks) would be compatible with these simultaneous actions, while others (e.g. aimed ranged attacks or spells) might not be "parallelizable." In the latter case normally simultaneous things like movement cannot occur during the action, and if they have already started they end for the turn. For things like aimed ranged attacks or spells this is functionally equivalent to SMM. I also think it strikes a slightly better balance between game structure and descriptive freedom than SMM.
I also have an intuitive affinity for the notion that a person has enough attention to do one major thing and one minor thing each round. Reducing the number of actions also reduces the amount of disparity that can occur at the table due to some that optimize the heck out of actions others use more sparingly. Of course, limiting a round to one action would do this as well, but there are lots of things in the game that benefit from the smaller unit in my opinion. With only a single action one has to decide whether drinking a potion is a free action (in which case one pretty much has to say it can only be done once a round anyway) or a standard action (which would have pretty wide-spread implications). Actions are the single most important resource a character has in combat, so changing this with a dial is a tall order. Indeed, actions are so ubiquitous in the game that trying to support multiple system could also have a large impact on the published presentation of the game. This is perhaps the most difficult aspect of the game to modularize, and for the presentation issues alone I wouldn't be surprised if the game chose a single action economy and stuck with it, with perhaps a few very minor options. (I'm otherwise quite optimistic about the potential for modularity to work.)
Finally, a mathematical curiosity about action systems. Suppose one wanted to use an action point system in D&D. That is, each round there are a given number of points to spend and every action uses a specified number of points. Maybe the goal is to allow a more nuanced spectrum of fast and slow actions, that doesn't really concern me here. However, for balance reasons suppose one also wanted to define a baseline number of points for these actions in order to preserve the idea that one can take at most a single "standard action" in a round, at least but never more than 2 "move actions" in a round, and at least but never more than 3 "minor actions" in a round. This system with those conditions can be represented as a constrained linear system, and it turns out that this system has no solution regardless of what numbers one might try. In other words, the fundamental behavior of SMM cannot be reproduced in any action point system and vice versa. This also means that any attempt to place literal times on actions (e.g. a standard action is 4 seconds, etc.) without also allowing that some actions have a "simultaneous" component will necessarily fail. (Not that people usually try to interpret actions so literally.) Given the sequential nature of SMM, I find that quite interesting.
