Great characters, imaginative landscapes and creative questlines bring back some of Morrowind's magic.
The whacked-out Daedric realm is begging to be explored and rewards wandering possibly better than any other Bethesda game besides Fallout 3. Each location -- whether it's a town, dungeon, ruin, camp -- has a bit more vivid unique personality than Oblivion's somewhat cut-and-paste approach to populating the world. You can tell the designers had a blast writing the game, as the standard conventions of fantasy RPG quests are thrown out the window and you dabble in a main quest and side-missions as good as the base game's much-loved Dark Brotherhood questline.
Game mechanics are identical to Oblivion, which is a little disappointing. Oblivion never really had a compelling leveling mechanic, with no new powers or abilities to dangle under your nose as you progress. Combat plays the same at level one as it does at level 20. It would have been tough to introduce significant mechanic changes in Shivering Isles, but at least several spells give you a way to mess around in the world after you play through the main quest.
You can easily spend 20-30 hours exploring the Shivering Isles, which feels like a godsend in our current age of 10-minute DLC add-ons.