Detailing that doesn't convey what is/is not a walkable surface. The textures used or the layout and context of the jump should convey what surfaces are have teles and which do not.
Tele destinations that are too close to the start of the jump. I like to walk forward a bit after failing a jump to get a kind of 'ramping up' feeling when going for the jump again. Generally a tele destination shouldn't be less then 128 units from the start of the jump.
Tele destinations are more then 1 unit off the ground when the jump doesn't require it, or tele destinations that are 0 units from the ground.
A good example of a jump that has a poorly placed tele destination is the crouched bounce to triple on Christmas. It's a good 8 units off the floor, and only 64 from the start of the jump. Every time I fail, I need to wait until I hit the ground, and back up a bit before walking forward and trying again. Which gets old quick cause I fail that jump alot.
When the starting teleport for the jump is at the same height as the part you walk on, unless the jump is a pogo. jump_steel is a good example of this one.
When a map doesn't have a secret or bonus jumps.