(This last bit raises issues of tension between roleplaying a character's decisions and making informed player choices, but this tension really applies to rules master as well: Can you really justify your character taking that highly-optimized feat combination?)
The place where games with more complicated rulesets can shine is in tactical options. (I don't mean just combat tactics here, but that's the most obvious place where they show up.) That's not to say that old-school games lack tactics - one of the advantages of such games is that players can try just about anything. Crazy ideas are often encouraged. What old-school games lack, though, are systems for tactics. Crazy ideas are often encouraged... but not always. If the GM thinks (correctly or incorrectly) that something is dumb, he can (and often will) smack the attempt down. There is, of course, a lot of variability here. Some GMs take great delight in "laying the smackdown" on what they see as dumb player choices.
What do systems for tactics get you?
First, it tells players that they can do this sort of stuff and gives them a rough idea of how effective it would be. Let's say I wanted to try to trip a giant. In an old-school game, the GM would decide if this was even possible. I might, then, get a random chance of success. It might be a better chance if I have a good plan. Still, it is unlikely that any two randomly selected GMs would give me the same odds of success. One might just rule it impossible. Another might think the idea was cool and give me a 75% chance of success. Contrast this with d20 games, for example, in which I would have a pretty good (though imperfect) idea of how likely I am to succeed... and whether success is even in the realm of possibility.
There's something to be said for that.
On the other hand, there are drawbacks: enumerating tactical options implies that these are your options. Creativity in tactics isn't necessarily encouraged here. Instead, you are encouraged to choose from the options given.
Perhaps more importantly, there's a lot of overhead in terms of rules to learn. I happen to like achieving rules mastery, but a high learning curve means that I have fewer people to play with and I spend more time teaching people how to play and looking up rules... when I could be playing instead.
So... is there a sweet spot? Simple rules that offer a tactical system that promotes creativity?
I think there is.
There might be rules out there that would do this. I'm not sure. There are certainly some that try: Savage Worlds and Fate come to mind. Fate , to me, comes closest to a system for creative tactics... but it still isn't quite there.
I feel like I'm groping toward something. Maybe it is a stripped-down Fate with a tweaked maneuver system...
If there are games you think hit this sweet spot, leave a comment and let me know. I'm wondering whether I missed something obvious here.