I particularly admire the quality of his mimesis: O'Brian achieves a high degree of verisimilitude in the characters' speech and opinions, and does not sugarcoat or censor the less than savory aspects of late-18th century life. For instance, one shore leave for the sailors after a successful voyage does not end in harmless drunkenness, but in multiple cases of rape of women in the port town.
The main characters are interesting, and Stephen Maturin in particular is an intriguing and highly original character.
The language is very rich but never tiresome; be prepared to crack open your trusty dictionary every other page or so. I found the language contributes greatly to the color and character of the narrative.
Thanks to all my friends who had read and recommended it in the past: Tal Cohen (as early as 2000!),
avva, gaal, ygurvitz, and probably a few others I forget.