This is an interesting idea that you're raising in mentalism. I think for me, it's important not to think like a magician per se when you're talking about making something more visual in mentalism. Mentalism is inherently theatre that is in a person's mind ... so the visual aspects should somehow illustrate as best as possible what is going on in a person's head.
I think part of what makes mentalism visual is the staging or theatre of an effect. I remember in Banachek's PSI series, he discusses this idea of making mentalism more visual. But Banachek goes on to say that there is something already visual or "odd" about seeing someone standing on stage with a blindfold on their head. Seeing something that is out of the norm is already visually interesting. Although interpreting "visual" to mean a large prediction banner on stage isn't a bad idea, I think if I were trying to make my mentalism more visual, I would be thinking more in terms of staging and composition.
Does the visual language of my stage tell the story/effect I want to.
Suppose you had an effect where you needed to set the atmosphere of a seance or perhaps give the impression of a casino ... have you been able to set that mood? Do you have the appropriate prop? Derren Brown's "Mystery Box"
http://youtu.be/inwAb0n4Mto effect in his Enigma show comes to mind. You can see how visually interesting the stage is set. There is a clear composition of the prop, the subject and the mentalist.
My point is, you don't need to be visual in a magician's sense if you're doing mentalism. The visuals need to make sense for the effect you're doing. In the Brown example above, visually the boxes and the ball of yarn being pulled back all give the illusion that it is impossible for this medallion to have been inside this box originally.