I had a GM Lumberjack in UO, 4 level 55+ chars in EQ (60 War, 60 Cleric, 59 Druid, 55 Enchanter), beta tested AO and AC, 2 Rank 5s in Shadowbane, a level 30/15 thief/nin in FFXI, and a level 18 controller in City of Heroes. So I can honestly say that I've been playing mmorpgs for a decent ammount and at the same time I can say that World of Warcraft is probably the most impressive one I've ever played.
Why is it supposed to be good? Well, the answer to that question is really the fact that a lot of people automatically associate Blizzard with quality games. The other part of the answer is that a lot of people really wanted to play Warcraft in an mmorpg setting. It was just one of those "wouldn't it be cool if..." scenarios.
Why is it actually good? The game is extremely polished even though it is still months off. This is practically unheard of in an mmorpg (every single one is always a public beta). The classes are all balanced quite well, have unique roles but also have a lot of flexibility. All of the quests are engaging and fun, with enough changes to keep you interested and above all keep you moving around the world. The trade skills that I have seen are all involved while not creating a new career for your own life. Unlike EQ, you shouldn't need to be farming 6 days a week to craft for the last day. The transportation system is extremely well thought out, allowing you to quickly return to areas you've visited while not making it too easy for you to get where you shouldn't be at your level. They have a flawless implementation of instanced dungeons, you can even log out in the middle of them or run back and re-enter them (within 30 minutes). WoW also appears to have the most solid PVP since UO. Albeit, it isn't as lawless as UO, it will be a lot of fun when the game gets going.
To understand why what I have listed makes this game good. You have to look back on why other games sucked. Why Shadowbane got boring after 3 weeks, why CoH doesn't have more than 4 different dungeons, why EQ requires you to sit on your ass at the same place for 7 days straight to get 1 item, why UO had more cheating and macroing than any other game combined, why you could fight the same crab monster with a different name from level 1 to level 30 in FFXI (I sh!t you not), or why AO was basically 1 mission in an infinite loop from start to finish.