What's good about WoW? It depends on how much experience you have with MMO's.
-Character Development: I found myself getting extremely attached to my WoW character. I love playing him, leveling up, learning new abilities, working on my professions, etc. In most other MMO's, I tend to get bored with each character after a certain point, and start another. WoW really encourages sticking with one character to see the extent of its evolution.
-Quests: WoW has a great quest system. NPC's offering quests at your level will have an indicator over your head, and when you accept a quest it logs it for you in your quest journal. You don't have to guess about where to go, or what to do. For the most part your journal contains all the info you need. The quests also give you all the gear and money you will need, plus experience towards your next level as you do them.
-Gameplay: The game is immersive at times, but doesn't require the time commitment other MMO's do. You can jump in and play for an hour if you like, or go on a level grind for hours at a time. That's a big deal to people like me who don't always have a lot of time to invest in a game. The combat is also a lot of fun.
-Graphics/Design: The graphics, while a little on the cartoony side (in the same vein as Warcraft III), are breathtaking and vivid. The sights and sounds of WoW are extremely well done, and the level designs on the different areas is amazing. You'll really remember the first time you visit the Dwarven city of Ironforge, or the Human city Stormwind. Every little thing in the game design is geared towards ease of play. It's intuitive and I had no problems jumping right in.
Crafting: The crafting system, which is based on professions, is remarkably easy to get into. It's fun working on your skills, and learning how to make new items that you can sell or use on your character. I've never had so much fun blacksmithing in a game.
I can't say enough good things about it, really. Anyone into MMO's or the Warcraft universe should definitely give WoW a try.