Alastair Reynolds 'Chasm City' is better written than 'Revelation Space' (though the first is still a thumping good space opera). I'm not certain whether or not it counts, though, not being a direct sequel.
Maybe a better example would be 'The Confusion' - the second book in Neal Stephenson's Baroque Cycle. It lacks the stylistic jumping about of 'Quicksilver' (which I enjoyed, but which kept breaking the sense of immersion), and has more 17th-century cryptography and bits getting chopped off.
Many of the sequels to Harry Potter and the Philosopher's Stone were better than the original book. I think that is because the tone and style grew up to suit the audience, who was growing up with the characters.
Yes! Sometimes if it's the author's first work then you can see their development through the pages. Or even if it isn't their first work, sometimes the first novel in the series is almost a set-up for the rest. Simon R. Green's Nightside books start out typical urban fantasy with a supernatural noir detective, once he builds up the universe however it becomes an excellent series with almost an ensemble cast of unique and interesting characters, but it takes until about book 3 or 4 before things take off.