What specific characteristics made Pluto's status debatable before 2006, even before Eris?

Answer

Small size, highly elliptical orbit, and many neighbors

Pluto, added to the roster in 1930, always occupied an ambiguous position within the pre-2006 planetary framework, even prior to the discovery of Eris. While it was generally accepted as meeting the unwritten criteria of orbiting the Sun and possessing visible mass, its specific orbital characteristics stood in stark contrast to the eight classical planets. Pluto exhibited a highly elliptical orbit, which deviated significantly from the nearly circular paths of the larger worlds. Furthermore, it was recognized as being substantially smaller and less massive than the giant planets. Perhaps most critically, Pluto resided among a population of other bodies in the outer reaches of the solar system (Kuiper Belt Objects), meaning it had not dynamically dominated its orbital zone, suggesting it was an outlier compared to the established planetary group.

What specific characteristics made Pluto's status debatable before 2006, even before Eris?

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