Right Thought is thought that supports Healing and Connection. Right thought is thought that supports the expansion of consciousness into the physical unit. Right thought is thought that is in perfect alignment with one's own Resident Monadic Self, Highest Self,or "soul" if you prefer.
Right Thought and Right Action are particularly important when it comes to the process of aligning the physical unit with its resident monadic consciousness or "animating spark." Since alignment of the physical unit the resident monadic consciousness is an essential prerequisite, establishing right thought (not to mention right action) is a critical precursor to unfolding of consciousness into the vessel.
Poltinus refers to the importance of establishing proper thinking when he says "Our thought cannot grasp the One as long as any other image remains active in the soul." He further suggests that to grasp The One you must "set free your soul from all outward things and turn wholly within yourself, with no more leaning to what lies outside, and lay your mind bare of ideal forms, as before of the objects of sense, and forget even yourself, and so come within sight of that One." Personally, I disagree. For sure we should lay bare our mind of ideal forms (i.e. archetypes), but getting rid of them altogether doesn't make any sense. I would suggest, and I have, that if we are to grasp the One we must replace Old Energy Archetypes with New Energy Archetypes so we can have a clear and organized vision/understanding of that which we "grasp" at. I also don't agree with the old "turn wholly inward" thing. Unless you got slaves to tend to your needs, as Poltinus probably had, that's an unreasonable, and unnecessary, spiritual requirement. I also think that grasping what the One has to say is only a little thing, like when you realized, as a very small child, you had a nose on your face. Let it titillate you for a while, but get on with your own life's purpose, whatever that may be.
On the Lightning Path, right thought is the equivalent of the Jainism's samyak darshan (right perception) + samyak jnana (right knowledge). On the Lightning Path, right thought is a combination of right perception and right knowledge.
The Zoroastrian Gathas open with statements about "good thought", good words, and good deeds. "I praise good thoughts, good words, and good deeds and those that are to be thought, spoken, and done. I do accept all good thoughts, good words, and good deeds. I do renounce all evil thoughts, evil words, and evil deeds. " GBd Introduction, 4
GBD 11. 17 "I celebrate my praises for good thoughts, good words, and good deeds for my thoughts, my speeches, and (my) actions. With chanting praises I present all good thoughts, good words, and good deeds, and with rejection I repudiate all evil thoughts, and words, and deeds"
Sharp, Michael (BOOK3). Lightning Path Book Three: The Basics. Lightning Path Press. 
Sharp, Michael (GA). The Great Awakening: Concepts and Techniques for Successful Spiritual Practice.