Looking at the above posts, I suspect the difficulty has to do with how much math is involved.
I generally think the hardest mental arithmetic anyone should have to do while actually playing a game is a maximum of double-digit addition and subtraction (for percentile systems), or single-digit multiplication. Anything harder than this breaks my immersion because I have to think about it too much. (I don't mind character design systems that require more complicated math or formulae than this, as long as it's all "front loaded" to be done before play starts.)
I'd be curious to get input from others about whether they think this tolerance is on the low or high side for gamers in general.
I'm naturally inclined to push the envelope on such things. Math is not something that slows me down much, usually, until it gets into things no sane person would put in a game (and even then, it was because I'm rusty on some of the advanced stuff). My group varies from "generally good at basic math" to considerably better than that. Occasionally, I've had a person try to play that struggled even with basic addition (due to youth or lack of math interests), but for whatever reason either the player doesn't last or they get better at basic math really fast when playing. The latter aspect of slightly challenging math improving the audience shouldn't be underestimated, even if it is a tertiary consideration.
Yet, I push hard now for simple as I can get, scraping out every little edge of simplicity while still meeting the goals of the design or the mechanic, rule, or house rule. The reasons are primarily two things:
1. Accounting. Almost everyone can balance their checkbook. Almost no one enjoys it. It's a chore. When advancing your character or designing a monster feels like accounting, it's draining some of the fun out of it. Then you get the distinction between what
can be done out of the play session versus what
will be done out of the play session. Fantasy Hero is still as fun to run and play for me as it ever was. Preparing? Freaking nightmare of tedium. That's coming from someone with so much system mastery that I could design 250 point monsters to the decimal point without opening the book. High level D&D 3.* pushes the same buttons on prep (though I don't enjoy playing it, either, for related and unrelated reasons).
2. As I have gotten older (and along with me, some of the players, though we keep having younger ones join), it has become apparent that the math handling we can do
automatically at the start of the session can be a bottleneck at the end. It's notable at the end of a 3-4 hour session, if you pay attention to such things. It's inescapable to even the most clueless observer at the end of a 7-8 hour session, even with plenty of breaks. Which can be a problem when you are ending an exciting fight. Given busy lives, some people show up tired. There's an obvious hit when that happens, too.