As I haven't played the game, I'll just construct a specific scenario to hopefully explain what good design means to me:
Imagine a small village with lots of enemies. Your goal is to escape to the other side.
You can try to attack them, but due to their sheer numbers, you will get overwhelmed quickly unless you are way overleveled and/or spam potions.
So instead you decide to watch the enemies a bit, their paths and soon discover spots where you can slip through undetected with clever use of cover. Mess up and you will get slaughtered.
In short: The game offers you reasonable options to handle a situation within it's own mechanics without expecting you to exploit its weaknesses.
Now an example for bad design:
Imagine the same small village with lots of enemies, but without the cover to sneak past them. So the game apparently wants you to fight them all at once or try to run. Both will certainly get you killed unless you leveled up like crazy before (Let's say this is one of the earlier quests), so what are you going to do now?
After some Trial & Error (Which is nearly always a bad thing), you discover a flaw in the enemies' programming, namely their limited "aggro"-ranges. So you pull them out one by one, until there are only a few guards left so you can just run.
In short: You only win because the AI is badly programmed and/or plain stupid - and the game wanted you to abuse it.
@SD:TEHO
Not really, I think.
If a game's intended way of playing (I'm assuming now that exploiting the AI isn't) is so (deliberately?) infuriating, that abusing the weaknesses in the script/AI/Whatever is the only way to have fun, then something is
very wrong.