He’s taken to the time to write up a lengthy developer’s blog. In a word: lighting. Virtually every aspect of the visual revamp of Spellplague Caverns centers around the dungeon’s lighting. As Poage put it, he basically turned off all the lights, then turned them all on one by one, tweaking them all in the process. The results are pretty spiffy, if I do say.
But you can refer to the dev blog for all of this in its glorious detail.