Well, so there are 70 levels - but getting to the top level is only the beginning (usually). It takes a fair bit of time just to reach the max level, so if you don't know anyone who currently plays who would want to help you - it could be somewhat lonely / boring. But not necessarily - I leveled all the way to 70 by myself (after the friend I started w/ bailed early on) and then switched servers to be with some real life friends.
You say you enjoyed WC and SC - but did you like playing the single player missions more, or did you do the player vs. player (PvP) online? Either way, you can do both in WoW. You can either join with other players fighting against progressively tougher bosses that require groups of 5, 10, or 25 players to team up to kill. Or you can do PvP and fight against other people in real life (in teams of 2, 3, 5, 10, 15, or 40).
The main difference is instead of having control over all your forces (which is good and bad - since although you know what you want them all to do in your head, it takes awhile to express that thru clicking and whatnot) you have only yourself. Of course, you can do a heck of a lot more things.
Plus there's 9 different types of classes you can be, and great variety within each one, so there's plenty of options for being able to do different things if you get tired of what you are.