Often occupying the space between miniatures games and RPGs and as an avid player of both, I think there is a lot the two fields could learn from each other. 

At the risk of oversimplifying a little, I am going to list a few that I think are at least worth considering.

A: Write rules assuming the player will use them.

When faced with a rulebook that is perhaps less than stellar, the answer is often that the GM is expected to modify or even cut portions of the rules during play. I think RPG writing could benefit from a firm stance that the player is expected to use the rules as they were written and in their entirety. Not because this is a better way to play, but because it enforces some discipline on the writer. If the game is too complicated for a group of players to execute in play, then go back and make the changes you need until it can be played comfortably by the expected target group. 

I suspect there are two things at play here. First the presence of a GM to paper over problems has become a bit of a crutch to lean on and secondly a lot of RPG play does not engage the mechanics heavily. In a miniatures game, the mechanics are inescapable and each player must engage with them, which tends to ferret out problems rather quickly.

B: Separate rules and fluff text.

While some wargame books have plenty of purple prose, for the most part the actual rules aspect is clear and straight forward in the text. RPG books are fond of mixing narrative descriptions and mechanics in the same text block, making it hard to actually reference the rule at a glance or, even worse, making it difficult to understand exactly what the rule does.

C: Unique terminology

If a wargame set uses a term like Suppressed, it usually will only refer to a single specific status in the game. Either a unit is Suppressed because a rule has said it is, or it is not Suppressed.

While games with a lot of status effects tend to avoid this trap, it is not uncommon to find games using terminology either in an unclear fashion or using multiple terms to refer to the same thing. Is a wounded, injured and damaged character the same thing? Does a spell that heals injuries also heal wounds? Ideally these things are clear cut because the same term is always used (and is then not used anywhere else).

D: Take distance seriously.

Now I rarely use miniatures in RPGs myself, but when I do I often found the rules are incredibly vague in a way that just would not fly in a miniatures game rulebook. Can my character move through a space covered by another character? What happens if I move through a space occupied by a prone character? Can I move diagonally? Do characters block lines of fire? 

(As a separate note, more RPGs should, in my opinion, at least consider measurement over squares but that is a post for another night). 

I am sure I can think of more (such as having playtesters actually test the mechanics) but this will do for now. In the future, a few things miniatures gamers could learn from RPG players!