Any take-off will have to have a transition from the angle of the run-in to the angle of the take-off. The kind of jumps kids build, with bricks and planks of wood, have an immediate transition, from say 0 degrees to 20 degrees as you hit the plank. Obviously if you have a nice flexy plank it smooths this transition and gives more pop, every 9 year old BMX expert knows that.
On a beautiful dirt jump the angle will go from 0 degrees to close to 90 in a very gradual fashion, this is what makes a good jump smooth. Similarly, a nice landing will take your forward motion and the rate you are falling at, and deliver you smoothly onto the horizontal plain once more.
In the main, all you need to know is that if you don't fancy the jump, scratch your chin, shake your head, and knowingly say "I'm just not sure about that transition…"