What kinds of things are you talking about? Physical things? Situational things? Mental blocks?

I know that one universal block to LDing is stress. Stress, lack of motivation, lack of practice, lack of consistency, and giving up too easily. The one thing that hurts the most is when someone tries for a few days, but gives up because it's not happening yet.

Physically speaking, poor nutrition, less sleep, stress, poor health, and lack of awareness (going through the day like a zombie) is detrimental to lucid dreaming.

Not keeping a dream journal is bad. Pretty much anything that hurts your quality of sleep (like an uncomfortable bed).

That's all I can think of for now.