Easy Mode: This is basically just a tutorial. Not many experience points are needed to advance from level to level, but you can only reach a maximum of level 30. You cannot create or join a guild, and you get no access to any Talent points. You also are limited to Common and Uncommon items -- you cannot use any Rare, Epic, or Legendary items.
Normal Mode: This is where most players will start off. You need an average amount of experience points to advance (something like what we have today). You can create and join guilds normally, and you also get Talent points. In addition, you also get access to Rare items.
Hard Mode: This is for more hardcore players. The amount of experience required to level up is doubled. In addition to all the benefits of Normal Mode, however, you also get extra stat points to each attribute every time you level up, as well as a five extra Talent points to spend however you like. You can also make use of Epic items.
Ultimate Mode: This is for people who live, breathe, and die WoW. The amount of experience required increases exponentially from level to level, so that, for example, leveling from 69 to 70 requires the same amount of experience required to level all the way from 1 to 69, and so on. However, you get twice as many bonus stat points each time you level, and you also get 15 extra Talent points to spend. You can also make use of Legendary items. But! ...and this is a especially fat "but"... if you're character dies, you have only 3 minutes to be resurrected -- no ghost runs, and no graveyards in PvP.
Thanks be to Blizzard that WoW does not use this system or anything like it! It would be a horrible idea to segregate out players even more this way, though I do think there are some people who would like it. Also, if these "modes" were actually in place, the number one goal of most players in the game would be, not to become Ultimate players, but to make Ultimate friends, as well as to take down Ultimate enemies on the opposing faction. It would be a big achievement for the winners, as well as a sad, sad day for -- all too literally -- the Ultimate loser on the other side.