Game design questions - I agree 100%. Physics questions - don't worry about it. Not that important.
What I hear you saying is that players need enough information to make meaningful decisions, and they aren't really doing that if choosing to climb a tree is effectively suicide. No dispute there.
I also hear the answer to the question: how much do players need to know?
A: Players need to know, at least, if they're playing a game in which climbing a tree in battle is a bad idea.
More specifically, they need to know if climbing a tree is a bad idea
in context. The context being: the system, the situation and their character. Climbing a tree is likely a bad idea is the system strongly punishes your defenses while doing so. Next we must consider the situation, your defenses are being punished, but is anything actually threatening to hit you? (perhaps a flood is coming, not a hail of arrows). If nothing is going to hit you while your defenses are down, punishing your defenses is an irrelevant point. Finally, we need to consider the sort of character you have, perhaps you are a sort of cat-person, and you get a bonus to climbing trees.
This is what a character needs to know in the situation: first, the rules around "what are the consequences for climbing a tree?"
-There is a massive difference between the system saying: Your AC is reduced by 5. And: When you climb a tree you trigger a free OA from every archer within line of sight.
Those two items have now determined what the player needs to know next. What does a -5 AC mean? Is it the difference between being completely defended and being completely open? Or is it a minor change, because AC ranges from 10 to 45. Bringing your AC from 40 to 35 is a far less meaningful change than bringing your AC from 5 to 0. If say, climbing a tree provokes, they need to understand what they're risking. What constitutes LOS? The player needs to know. What is the range of an average bow? The player needs to know this to determine if making himself vulnerable is worth the risk. What is the damage of the average bow? The player needs to know this to make a risk assessment. Not being able to make a
reasonably informed judgement call leads to players thinking their judgments are meaningless.
Finally, the character needs to know if they're actually any good at climbing trees. Maybe they're half-cat? Maybe they have high skill points in climbing. Maybe they have a special talent that lets them climb trees without disadvantage.
Knowing these things adds consistency to gameplay, just like knowing certain things adds consistency to real life.
Can you change a tire? I can. So when I need to change a tire, I can assess the situation, get the required tools, and get the job done. I don't have to ask my dad if I can change the tire, I
know I can change the tire. Maybe I'll need to make one strength check to pop the lugnuts off, but I won't have to make an intelligence check to even know what I'm looking at. People who know what they can do take definitive actions.
Can you play football? I know the basic rules, and I'm not very strong. So while I
can play football, it might be reasonable that I need to make some strength or endurance checks. I might need to make some intelligence checks to understand the rules. I am unskilled in football, so I cannot take definitive actions.
So, here's my question to you, [MENTION=42582]pemerton[/MENTION]: since every game has rules, and it's highly likely that the majority of these games will have rules affecting the outcomes of combat, are you saying that players always need to know the rules of the game? That there are no situations in which it's okay for the players to not worry about the rules? (Not building a straw man here, just trying to clarify.
It is sort of a straw man though. You're using absolutes. I know I'm not Permeton, but still your question is couched in protective language. Not worrying about the rules is different from not knowing the rules. I know the rules of multiple systems and there are often times when I don't worry about them because of the Rule of Cool or whatever other reason. I'd argue
knowing the rules makes it easier to not worry about them, since knowing them mean you can determine when you benefit or not from their lack of application. It lets you determine when the lack of consistency in rules/rulings has become detrimental to the game. Without it you're just left up in the air.
IMO: it is
always better for the players to know the rules. But that does not mean it is always better for them to concern themselves with the rules.
There are times when games without rules work. There are times when rulings are superior to rules. These are decisions the DM must make and these are also decisions that the players must make. They may have different answers as to when these times are and there's nothing wrong with that. But a player can't make a good assessment of a situation, of a game, of a ruling, without knowing the rules first.
EX: in my current "gonzo" 3.5 D&D campaign, the DM has allowed casters to have iterative spellcasting, just like fighters have iterative attacks. Does this make casters more powerful? Yes, but the drawback is the eternal problem of novaing. It's very easy to burn through many spells quite quickly and then find yourself in trouble only a few minutes later. But I can make this determination because I understand the rules regarding iterative attacks, spellcasting and so forth. With that rules knowledge, I can make a judgement call about the situation on if I should take advantage of this to nova the bad guys, or not.
Knowing the rules has allowed me to make an informed decision regarding the actions my character takes.