Honestly i wouldnt call the WoW realm massive, SWG was massive (though mostly empty). Exploring is fun though
As for the warnings, they arnt there because Blizzard wants addicts, who will keep playing, paying, and especialy those that own 3 copies for alts and mules. Repetition is what makes it addictive. People keep playing to get that last set piece or other object, or that next rank in pvp, then a new and better set comes out so they spend months doing raids to get it, etc.
One thing that always troubled me with the MMO games is how carefully they are designed to addict you and keep you addicted, spending massive amounts of time in these games at the expensive of a life, or even your health. There have been several deaths, and other severe illnesses connected with WoW because people were playing these games for days at a time forgetting to eat, sleep, exercise, etc.
If you look closely at WoW you can see how it addicts and why it is so successfull at doing it. First the game requires a massive commitment of time to get anywhere in it, second the game is repeditive to the extreme (and repeating the same thing especialy if the brain finds it pleasurable will quickly lead to an addiction just like any chemical substance), third it has endless goals to achive, there is always something to strive for, and never enough time to do it all which increases the time sink, this furthers the addiction. People get sucked into itm loose all sense after a while, spend more and more time in the game, and some even confuse it with reality.
Now of course this doesnt happen to everyone, but most wow players spend far too much time playing the game at the exclusion of everything else, mainly because of the time sink.
You might also find that for the first few months you think this is a truely great game, but after a while your opinion may change, when you have been basicly doing the same things for months.