While there are certainly some bottlenecks, you can play at least 4 different characters (2 Alliance, 2 Horde) from level 1 to the level cap while repeating only very little content.
What Is New Content?
WoW currently offers eight starting areas (1-10/12ish) and probably seven paths to level 20 (both Mulgore and Durotar feed into The Barrens and Stonetalon, and I'm not sure there's enough content there to reach level 20 twice without repeating anything).
The bottlenecks Tobold is talking about start to kick in during the dreaded level 21-58 range. Some of the content is offered by neutral faction NPC's, like the Steamwheedle Cartel, the Thorium Brotherhood, the Argent Dawn, and the Cenarion Circle. In particular, around level 50 I suspect that the Tobold Challenge starts to run into real issues because so many of the traditional zones - Ungoro, the Plaguelands, and Silithus - are populated almost entirely with neutral faction NPC's.
Once you get through to Outland and Northrend, you can get a bit closer to the Tobold challenge; I'm pretty sure you could get through three characters relatively easily, since I'm advancing my Tauren through Outland currently doing almost exclusively Horde-factioned quests. Character number three could really focus on the neutral faction stuff, but I'm not sure that there'd be much of anything left for the fourth guy.
Then again, what exactly are we counting as "new" content? The Alliance guy wants you to kill Old Ironjaw for fur trim on a cloak, while the Horde guy wants you to kill the same wolf to contact the ancestral wolf spirits, but both quests are functionally identical - go hunting for the wolf in question in Terokkar and kill him.
Each faction does have its own signature quest lines (e.g. Thrall and the Mag'har, the Alliance reuniting Muradin and Brann Bronzebeard), but Blizzard has been going out of their way to have more and more content shared between the factions; either you're working for a neutral third party, or the two factions will have identical quests with different flavor text.
Why does this matter?
The big advantage to shared content is that more players will see it, allowing Blizzard to spend more time on it. A random quest in the corner of one zone that is only used by one faction really can't eat too much dev time, because so few players will use it. By contrast, if you're sending everyone from both factions through the same area, you can afford to spend more time fleshing things out. If you're only going to play one character ever, shared content means more, higher quality choices. If you're going to play a second or a third, you can repeat your favorites while still having some new stuff to shake things up with.
The problem arises if, as in Cataclysm, Blizzard is counting on players to re-roll repeatedly. In the short run, the plan of literally blowing up and revamping large chunks of the old world will certainly freshen things up for longtime veterans. In the long run, though, the question is whether we will see more content that, as with Outland and Northrend, is designed to be used by both factions. When Blizzard revamped Dustwallow Marsh, the answer to this question was indeed a lot of high quality but almost entirely shared content.
That's a trend that could ultimately hurt WoW's touted replayability in the long run.