System design; Do you trust the GM?

I've been playing around with several systems, as well as designing my own, and one thing that I've noticed is that some systems leave a lot to the GM, while some are completely anal retentive about explaining when a certain rule takes effect. Now certainly this links into G/N/S and rules heavy/light, but my question is how much of this is linked to the designer's expectations of the GM? When designing, or choosing a system to play with, what are your expectations of the GM's ability/style of handling a system, its rules, modifiers, and math? What are your expectations of players? Do you expect certain rules will be ignored/never used("let's all roll saves for each one of our items after falling in a pit, modified by your body weight and encumberance!"), modified("We just always assumed you started with max hp"), or to take precedence over the other rules("Alignment, feh! What's my THACO?")? Chris