I second the Red Mars/Green Mars/Blue Mars Trilogy. Although I think that they got worse as they progressed. I don’t think I finished ‘Blue Mars’.
I second the Julian May ‘Galactic Mileu’ and ‘Pleistocene Epoch’ series. Good worlds, kind of disappointed me at certain points, but really good nonetheless.
I also second (or third?) ‘Ender’s Game’ and ‘Speaker for the Dead’. Also the recently published ‘Ender’s Shadow’. Much of what Orson Scott Card writes is very good. I liked most of the ‘Earthfall’ series. His novelization of ‘Abyss’ is one of the few books based on movies that really works.
By the way, if you liked Diamond Age try Snow Crash (you’ve got to love a serious novel that has a main character called Hiro Protagonist) and Cryptonomicon. Cryptonomicon is not exactly Sci-Fi, but I think it appeals to similar crowds. (I started a thread called The Neal Stephenson Appreciation Thread.)
I really enjoyed the two Uplift Trilogies (the two series that Sundiver was the first part of) but I recognize their weaknesses. I read later that Brin was trying to do an homage to the old planet-smashing space operas, and I buy it (though Sundiver is more of a mystery that shares the same setting as the later books).