In my opinion any discussion on tactics must be firmly grounded in a knowledge of the rule set that you are playing under. It always amazes me how many people I play against don't really know the rules of the game. Sometimes even the most basic rules of the game. I'll be the first to admit that there are some rules I am not super familiar with, mostly because one never seems to run into them in a game (things like rules for bunkers, or how roads effect vehicles.) I have a rough idea how these rules work, but see them so rarely that I would not stake my life on knowing the rules. However, knowing things like how far your models move, how to shoot during the shooting phase etc.. are important to know.
First and most important for anyone to learn is how the rules apply to their chosen army. Say I choose orks, I would feel that it is important to know what I have to roll to hit in shooting. What my WS is and what I usually have to roll to hit in CC. How far my models can move. What furious charge does...and so on. It seems to me if you really have to put a lot of effort into remembering the rules for a unit, or just the rules in generally it becomes much more difficult to use said unit tactically no matter how good said unit is, especially if you are playing in an environment where time is an issue.
As an example lets look at the Vendetta, a unit that is pretty universally considered an excellent unit. While one could argue that this would be a great unit no matter who uses it, knowing things like say the range of your weapons (without having to look when shooting comes up), knowing how the fast vehicle and skimmer rules work (so you don't move flat out into terrain and kill yourself). Much of this may seem to be a no duh kind of situation but, people consistently get rules like these wrong, or at least don't know them well enough to think about how best to use said unit.
For me it comes down to this...if you are dwelling on the how do I do something, you don't have time to think about the why. If I need to think hard to remember how something works I cannot plan for it tactically.