Red John seems to be a narcissitic and really intelligent Psychopath.
Wikipedia says the following points are typical characteristics of such psychopath:
-Glibness/superficial charm
-Grandiose sense of self-worth
-Pathological lying
-Cunning/manipulative
And thats exactly how I and I think most of you see Red John.. No weaknesses, a bit intimidating, powerful.
It would be a dissapointing finale if a person like Bertram, Minally, or also Brett Pettridge (he can't be red john: http://youtu.be/k26u5EOWWTU ) was Red John. The only person having those charcteristics so far is Brett Stiles (and Jane but he isn't red john; would be too dissapointing, too).
There are enaugh episodes with him that the audience will know him, he is a good friend and competitor of Jane and Jane and Bret know each other for longer.
Thats of course no proof that he is red john but maybe proof that the other suspects are unlikely red john.
:D