What makes a good motif?
By definition, a motif is characteristic and memorable.
Internal contrast is a motif’s most important characteristic. belkin-2018 (p.1 par.1)
A motif becomes identifiable through repetition - sometimes quite literally just repeating the same unit, but more often than not, variations are applied.
Motifs are typically a couple notes long - they can’t be too short such that they’re basically “anything” but if they’re too long, the listener finds it hard to recognize it throughout a composition.