Like Aqua I tend to regard X6 as a good example and Zero/ZX as a bad one. A challenging game to me is one that remains challenging even when fully equipped. To me, ZX and ESPECIALLY Zero are not Hard, they're Handicapped.
I don't enjoy knowing that my opponent is pulling any punches either, which is perhaps the one imperfection that haunts me on ZX. It is not physically possible for a fully powered player to fight a fully powered boss. That there's a lousy design.
It should also always be realistically possible to not die. Note that I said possible, not likely, there's a difference. I'm no stranger to losing lives, but the ability to react should count for something before a life is lost. MM9 is a bad offender in this category, near the beginning of Splash Woman's stage you're dropped directly into the middle of a 4-spike-wide divider that you cannot see until the screen finishes scrolling and you start falling. The trap is too wide and thus you cannot realistically evade that if you don't know it's coming, while it's completely skill-less to evade once you do, so the obstacle is senseless. The helicopter claws fall into this category as well; they drop almost instantly from off-screen with no warning, ever, and are always in the vicinity of spikes. More often than not you are automatically dead in such a case. The few areas that do have a chance for survival usually require hop-jumps that, once again, you have no warning of until it's too late.
The latter half of World Circuit S in Punch-Out!!'s Title Defense is another one (an otherwise kickass game, BTW). It gets to the point where reflexes simply will not cut it and you have to memorize the order of the punches your opponent will throw. If the only way you can win is to know what's going to happen before it does, then something's wrong.
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