"Finding Red John is like finding waldo in a children's book." 

I watched the Red Mile today on TV and after watching it I noticed something shocking. Dr. Steiner has a slouched posture, and when Red John saves Jane in the end of season 2, he also has a somewhat slouched posture.
Dr. Steiner matches Red John physically in that aspect.

Also, Dr. Steiner is fairly old. From the previous episode, "The Red Barn", we have found out that Red John started his killing spree much earlier than we thought. This means that Red John is fairly old, and this matches Dr. Steiner since he's old as well.

Moving on, we know Red John is a mastermind and can be deceptive. Dr. Steiner seems to be clueless through out the whole episode, but randomly in the episode he shows some great understanding of the mind and it's ability to cause hallucinations (signs of Dr. Steiner being skilled in hypnosis maybe? ). Regardless, Dr. Steiner definitely could be Red John.

Lastly, Dr. Steiner and Jane seem to like each other and Jane plays tricks on him and Steiner finds them fascinating, like a kid watching a Disney movie. He is very good at hiding his true identity, his Red John identity. Also, Dr. Steiner has Patrick watch him kill himself with pills. Maybe he did this so Patrick would never suspect him to be Red John.

Also, Dr. Steiner dies (fakes his death I assume) a few episodes before the epic season 3 finale. So now Red John is off the radar, and Patrick will never know. Here we are in season 5 thinking CBI Ron is red john or brett stiles, when clearly they can't be red john since they're too obvious, etc.

If Dr. Steiner isn't Red John, then I'd have to place my money on Walter Mashburn.