I think what I appreciate most in a first person narrative at its most playful and challenging (when well written) is two-fold. First of all, a psychological veracity or inner perspective mostly inachievable with an omniscient third-person perspective (this can be very simple - a tone and vocabulary distinct from an author's usual conventions). And part of this, yet also separate from it, is the deliberate creation of one or more unreliable narrators. Both of these aspects are on show in, for example, Mary Shelley's onion-layered construction of narrative in Frankenstein
, Conrad's Heart of Darkness
, Gene Wolfe's challenging writer-narrators in his Book of the New Sun
and the Soldier of the Mist
But I also love it when an author challenges or undermines the omniscient narrator too - I think the tones Una adopted at both the beginning of the Never-Ending Sacrifice
(semi-childish or confused) and Brinkmanship
(rhetorical) served to challenge the sense of writerly authority in each text well.