For most observers, what factor constitutes the single greatest limiter to achieving quality viewing results?

Light pollution

When analyzing the true cost and potential quality of astronomy observations, it is essential to distinguish between the price of equipment and the quality of the environment providing access to the sky. For the majority of people residing in cities and suburbs, the pervasive artificial light creates skies too bright to permit visibility of anything beyond the Moon, the brightest stars, and planets. Consequently, light pollution becomes the dominant constraint on observation quality, often requiring observers to spend time and money on transportation logistics to reach dark skies, a cost that can sometimes exceed the annual cost associated with owning modest beginner optics.