Starting his career with the famous Kana: Little Sister Visual Novel, Romeo already started grappling themes of taboo and outsider activities. His works thereafter have been about, as Jack Kerouac would have put it, "the mad ones... the ones who never yawn or say a commonplace thing". Kazoku Keikaku about an unconventional family, his magnum opus Cross Channel, about an unconventional school and a situation separate completely from social reality, Yume Miru Kusuri (which is not strictly his work but he supervised it) which is a 'problem novel' about various social issues, AURA, which was one of the proto-Chuunibyou work even before Kyoani and Steins;Gate, and of course Jinrui Wa Suitai Shimashita, which is about an unconventional society with an unconventional ecology. (All of these I read in translation, which is probably the worst thing I could have done. I aim to rectify this mistake in the near future) Most of these works have been applauded for a striking writing style which encompasses a wide range of concepts and vocabularies, as well as vibrantly abnormal characters, and a beautiful style of prose.
"Never say a commonplace thing" is just about correct, because most of the entertainment from Romeo is derived from his method of harnessing his splendid vocabulary to full advantage. Nisio Isin loves to have puns and a kind of sharp back and forth that revolves around a series of topics before hitting the crux of the matter. His work is a fully 'conversed' work and even the main plots of Bakemonogatari constantly involves interactions and interpretations over events. (It is quite fitting that the first monster of Bakemonogatari is a crab made out of words, because in Isin's supernatural world, it is the power of wishes, soliloquies and repressed desires that drives things, all unspoken words bearing monstrous weight) As opposed to that, Romeo cherishes the event, the sociological viewpoint, the psychological infatuation, the wry observation. You could say Jinrui wa is made out of a series of satirical thought experiments cluttered upon each other, but Romeo loves to hold back for that nice bit of melodrama and emotional catharsis. He loves to romanticize a strange unconventional viewpoint: the boy without an identity, the pragmatic lonely girl, a sentient AI's search for warmth.
But none of Romeo's jokes or traits are simply and can be easily summarized. He is a masterful wit, combining slapstick (the chickens, an entire series of variations based around slipping on a banana peel), satirical observation (thesis on how subcultures are formed), verbal turns (the cute-sy fairies saying some of the most horrifyingly insidious things, as well as the protagonist herself), and the well placed straight-man line (the protagonist's reaction after seeing processed bread). Like any great satirist, he makes the normal strange in such a way that we learn something from it. Who could forget the great line "Never hold meetings, the outcome is decided before the meeting begins"? The same kind of 'erudite expounding' for comical effect can be found in the best of Stoppard or even the highly abstract Beckett.
In any case the main flaw is probably, as I have heard from opinions on the work in novel form or on paper, is that other than humor, the emotional and touching aspect, the gorgeous imagery, the romantic outisder-against-conventionality sentiment, the scientific and philosophical intrigues, which eventually colors Romeo's work, is supposedly lost upon translation to a non textual form. Indeed without the conversations, which is his main touch, the show has basically nothing left to stand on. In fact the only main work of his that I haven't experienced yet seems to be his other supposed magnum opus Saihate no Ima, and the routes he wrote for Key's Rewrite (though I have seen some videos of the translation, and the prose does seem superbly spectacular).
What else can I say about someone who I have so little experience with? (I read all his earlier works waaaay before I could appreciate form and artistry, in translation, and now I hardly remember any of it) Romeo is a treasure trove that deserves to be plundered. His work probably deserves the same kind of attention the global Japanese superstar Murakami receives. He's at the forefront of the 'Visual Novel New Wave' (okay I have no idea whether this actually exists) with people like Meteor (whoever is out there read, and most importantly LISTEN, to the bewilderingly musical Forest, which is a work that straddles the line between musical collage and psychedelic fairytale) and SCA-JI, and he shows just how great a textual-audiovisual medium can really be.
