The Lorwyn/Shadowmoor blocks were all about the transformation of the populaces of that Plane after the Aurora (basically a 300 year long day/night cycle). I'm a big fan of the concept of moral ambiguity in fiction and fantasy, and showing the corrupted or redeemed incarnations of the tribes or races really piqued my interest. I like fallible characters, thus representing shifts in behavior and morality with the actual same characters and societies seemed like a really great take from an almost sociological standpoint.
All the races colorshifted, basically. In the canon, the faeries were the only race supposedly unaffected by the Aurora, but even they saw a color shift from U/B to U/W. Basically all the "good" races became corrupted and chaotic or evil, the "evil" races became somewhat neutral or good. In both the "day" of Lorwyn/Morningtide and the "night" of Eventide/Shadowmoor, races showcase both the good and bad aspects of each mana color. In merfolk, for instance, blue mana in day is knowledge, exploration, and control for the sake of order. In night, that same blue mana becomes covetousness, subterfuge, and control for the sake of theft.
If I recall correctly this is mostly the shift:
Kithkin: W --> U/W
Flamekin: R --> B/R
Elves: G/B --> W/G
Goblins: R/B --> G/R/W
Merfolk: U/W --> B/U/G
Giants- W/R --> R/G
Treefolk- G/W/B --> G/B
The blocks were a high point in MtG for me in terms of card power, diversity of function in cards, and the superb lore.