As a simulations, there are wasted time units. Among the people I know, i can have a very acute sense of time, IF I'm not performing a long task, otherwise I can pretty much guess closely how much time i'm consuming - this allows me to plan activities with hindsight.
What this does is gives another Framework for GURPS combat. Simplifying decision making processes so that people can make more tactical declarations in less time. Old DnD players have a framework to work with, and so does GURPS players.
6 second rounds - Assume 4 seconds of standard action and 2 seconds of move actions
Why 4 seconds of standard action? - Because ever attack has a full evaluate/aim build in. So every attack takes up 4 seconds.
Synergy - having 2 second move, makes sense because Move 30ft is = to Move 10 (which is Basic Move x2).
Synergy Note - if PCs always get max Aim/Evaluation bonus that's about +3 bonus in gurps, that is equal to around +6-+7 in DnD (+3 makes 50% to 83.33%; 30% divided by 5%). If you play with those statistics, GURPS guys playing DnD might allow multiple attacks at -7 (i recommend round down).
Attack of Opportunities in GURPS. There are no attacks of opportunity, unless the Player Tactically predicates the conditions for one. It is not limited to non-combat complex actions. AoO builds into a combat a predetermined list of Wait/Ready or Anticipatory actions. AoO actually makes opponents more tactical than they should be. A DnD Zombies don't exactly know a Wizard is casting a spell or there is a weakness in the stance of an opponent... they just attack what ever is in reach at the rate of attack they can muster.
Having no AoO, makes the game much easier... and even more tactical as players now must budget their time. If you budget 4 and 2 secs, you can simplify your decision making process. You can assign one or the other as "reactive" vs active actions.
Ex. I budget 4 sec as step and evaluate x3, plus step and attack. Then with my 2 seconds I would reserve for anything that would provoke an AoO/any time the opponent drops their combat stance. Note that the AoO is at -7 in DnD, or does not gain the benefit of Aim and Evaluate.In modern Combat the 6 second rounds carry over beautifully in my experience. It really captures the anticipatory and focused action in a way that players can't meta-game and interrupt themselves so easily and abort their current action. If you wait, you just wait - you can't pick and choose seconds. Modern combat is much more lethal, and thus focused action is a big deal.
Because of my GURPS experience, I will use a gurps engine within my DnD game. That means when players become more detailed in what they do, I will Hack DnD with GURPS. This is how I plan to build my Open Source Combat System - simple and elegant, with the doctrine embedded but with the "source code" available for GMs so that they can bring much needed customized elegance to their players.