Few things annoy me in games as much as those slopes that are just barely too steep to climb.  Are they there because of bad terrain design?  Is it a wall disguised as terrain?

I greatly dislike it when my character is forced around in a game.  Crowd control is one form.  Stupidity-based trap quests are another (can't I just shoot him now?).  And then there are the slopes.  You can walk for a little while down, and then it gets steep and pulls you.  You backpedal, jump, turn around and run up, crouch, stand, nothing works as the gravity inevitably pulls you downward.  This isn't a part of progress through a map; you can just run back around another way.  It's instead just an irritant.

Not as bad, but still bad, are the map edges.  With plenty of disk space and a desire to create a sense of place, developers like to put things beyond the edges of the map, usually more terrain and maybe some buildings.  The explorer wants to check them out.  If there is a wall, then they can see the wall, think it is lame, but recognize that not every game is Minecraft and so the map must end somewhere.  But the slope... maybe it can be climbed.  At places it can, though not all the way.  So you go up and up and one missed step and you're sliding back down.  Can I go there or not!?

I should be clear, a sufficiently steep slope becomes okay.  A cliff is clearly impassable and does not create this problem.