What I mean by Harn requiring a more intelligent type of gamer is that the materials read like a college level medieval history book. You need a certain level of education, experience and sophistication to understand all the myriad details and complexities much more than with, say D&D, which is written for the lowest common denominator. D&D catering to the lowest common denominator is not a bad thing. Why? Because it makes D&D more accessible, easier to grasp and (gasp!) even fun for those who enjoy that sort of epic, high fantasy game, and for newcomers to RPGs in general. I got my start with D&D, and am glad it was there to indoctrinate me into the cult of RPG geekdom. My chaotic young teenage brain probably could have neither grasped, nor appreciated Harn back then. Now, after two decades of experience, it is (actually, it was ready a long time ago, but CGI's distribution is so bad, I could never find any Harn products).
The level and depth of writing in Harn products is generally written to the highest common denominator, which makes it much harder to wrap your brain around if you are a newbie gamer or come from a "video game mentality" D&D background. 1e was mysterious and dangerous. 2e was not so mysterious or dangerous. 3e is neither mysterious nor dangerous in the least. It is like playing a video game, and all about power-gaming and multi-classing for benefits rather than RPing reasons. Everything moves too fast, from level advancement to BAB, to spell acquisition. I still enjoy 3e/d20, but am becoming burned out on this video game mentality. I want to reintroduce that sense of danger and mystery to my games. Harn helps me do that. I am using a tweaked set of d20 house rules mixed with Call of Cthulhu d20 Sanity and Magic to run HarnWorld. I do not own HarnMaster, though I may end up trying it sooner or later. I don't claim HarnMaster to be the best RPG ever, but HarnWorld is the best fantasy setting I ever read, and the only one to make sense. It's still not perfect, but it really does inspire an addictive, fanatical quality in its adherents, especially the newly converted such as myself. That's because it provides something other settings never have: Realism to the nth degree.
Read it for yourself and see what I'm talking about. Harn is not about farmers or sitting around bored or depressed. Harn is about realistic adventures with realistic outcomes in a realistic world. Yes, there are a few monsters roaming about, but they are logically explained. Yes, there are a few wizards roaming about as well (perhaps 100 in all of Harn, most situated on Melderyn, the Wizard's Isle), but they are neither all-powerful nor all-knowing, and not the overblown folly that is Elminster.

Harn is dangerous, because anybody can die at anytime from anything. Your uber-knight could still be taken out by a wily peasant girl with a kitchen knife, given half a chance. You never gain that feeling of invincible superiority that D&D engenders. You wake up knowing each day could be your last, no matter how rich or powerful or experienced you become. The laws of king and church bind and restrict you, spies are everywhere, waiting to whisper in the right ear and see you burned at the stake, beheaded or hanged. Harn is that first terrifying thrill you get when you realize you're not in Kansas anymore, Dorothy. Be afraid. Be very afraid!
Harn is not all doom-n-gloom, either. I don't mean to give that impression. People laugh, joke and sing as they go about their daily tasks. You could go your whole life on Harn and never see a wizard or monster (not even an elf or dwarf!), and most people never do. That's the way they like it, and that's why Harn is different. No elves and dwarves always underfoot, no rampaging, ill-conceived hordes of monsters, no 30HD fireballs from above. Because these elements are all kept to a minimum, they become that much more special (or terrifying) when they are encountered!