For new players (IME), the biggest thought-demanding mechanical decision is choosing your attack with an AEDU character. Their internal motivation is to attack a guy, but they have to step out of the game, look at their sheet/cards and pick the specific attack they want to make. Even with more experienced players, their decision is based more on the out-of-game list of powers (with the consideration of whether they want to blow a daily/AP) than it is on the in-game situation.
I don't find this very different, in the way it plays, from a D&D magic-user deciding what spell to cast, or a spell user in a power point based game deciding what spell to use.
In the fiction, the PC is wondering what manoeuvre to peform, or (in the case of daily/AP) whether to risk an extra burst of effort.
I think of this issue as one of immersiveness -- not in Monte's slightly strange use of the term, but in the sense of asking how much of the player's time is spent thinking about what is happening inside the fiction as opposed to thinking about the rules mechanics. The more the players have to step "outside the game", either to justify the mechanics or simply to select their next action, the less the players are thinking about the fiction itself.
I don't find hit points immersive at all. What is happening to my PC when s/he takes 4 hp damage from a dagger, or a magic missile?
I can intuitively record that x hp have been deducted from the character sheet while almost effortlessly imagining a corresponding loss of something to the character in the fiction.
Loss of what? Blood? Flesh? Mojo?
There was a whole RPG industry (from the late 70s to mid 80s) based on the fact that a large number of RPGers found hit points incompatible with immersion and verisimilitude. (I was one such.)
hit points and the d20 vs. DC system can sometimes produce results that are sufficiently counter-intuitive that they pull players outside the system (e.g. massive falling damage).
But these rules have the considerable merit of being very simple.
Runequest's d100, roll under is simpler than the d20 system. It's basic combat mechanics are a bit more complex.
Rolemaster's combat mechanics are pretty complex, but in my experience are also pretty immersive - because no one has to step out of the fiction and think about things from a meta-level in order to know what is going on. You just roll the dice.
The example I was thinking of is the tracking of health and injury. Almost anything designed to create a "more realistic" description of the topic than hit points involves making the game much more detailed and complex, which explains why D&D is still stuck on a videogame health bar.
If you go to Classic Traveller or Basic Roleplaying - where the health pool is based on the PC's physical stat(s) - you straight away increase verisimilitude without increasing complexity.
Jorune (I think - it's been a while) also had a simple health level system which increases verisimilitude without increasing complexity.
Classic hit points are ludicrously unrealistic, but in the heat of play you can easily overlook that fact, as the DM narrates grisly wounds and splattering gore.
Many aspects of 4e - hit point depletion, healing, forced movement, etc - can be handled the same way. These are all examples of what I mentioned upthread, namely, rules that distribute the authority to say what is happening in the fiction, plus an implicit constraint that those descriptions of the gameworld must abide by genre/verisimilitude constraints.