I don't think I've seen a movie as defined by its world and its language before.
The world was pretty much established by the production design in the opening pullback (a great "Okaaaaaaaaaay... uhuh" moment) and the rant by the homeless guy in the next scene.
The vitality of the language was unexpected (though I do recall that it was a defining feature of the novel, which I haven't read). Alex's mix of invented thug slang and 17th century eloquence contrasted nicely with the mundane talk of his parents and some other "establishment" figures.
The movie follows a clear three-act movie structure: Alex before, during and after his rehabilitation. Wonder if the book is as clearly delineated.
I think the opening act, showing how Alex and his gang revel in their depravity paved the way for movies such as Natural Born Killers.
Alex is a compelling anti hero: a monster fighting an even more grotesque system. The film is aware of the hypocrisy of evincing sympathy for Alex - it's part of the satire. On the other, due to McDowell's spirited, childish, cunning, pugilistic performance the sympathy works in its own right, even though this is a guy who gleefully rapes and kills and would (will?) continue to do so if it wasn't for the chip in his head.
Speaking of which - it's obvious to me now that Buffy's Spike owes a lot to Alex. Though Spike, even at his early, brutal best, is but a watered down version made palatable for TV consumption. (I wonder if Gary Oldman's Stansfield's love of Beethoven in Leon is also a tip of the hat.)