A little more than a year ago, we came across a post by Tobold (of the popular Tobold's MMORPG Blog) called "Horizontal expansions to vertical games," in which he explored the differences between "horizontal" and "vertical" expansion in MMOs. His blog was actually inspired by an earlier one by SerialGanker about horizontal expansions, pointing out Blizzard's apparent knack for "invalidating game content that subscribers have spent considerable effort to achieve," as he put it. The video game concept of "horizontal vs. vertical" isn't just about expansions; the notion applies to things like gameplay and progression as well.

In the context of MMO expansions and content though, "vertical expansion" is best described as expansions or game content that further the endgame by increasing a hard power cap (or maximum character level, in World of Warcraft's case). As Tobold pointed out, WoW is a traditionally-accurate example of the vertical expansion model; each of its expansions raised the level cap by 10, and the raid content released in-between always introduced more powerful gear. But for the first time in five years, Blizzard is developing Cataclysm—WoW's next major release—by using horizontal expansion (in part, at least).

You might be wondering what the deal is with "horizontal vs. vertical expansion" in the first place; is one form better than the other? What does it mean for the players? Quite accurately, Tobold offered the card game Magic: The Gathering as an example of horizontal expansion; a player's "power cap" doesn't increase with each expansion. Instead, expansions offer new cards (content) and game mechanics that allow the overall IP to stay fresh and engaging, without rendering prior content obsolete. In fact, players can still use content from the original game just as competitively as that released in any of its 50-plus expansions.

In MMOs, horizontal expansion is a similar model; instead of raising a level/skill cap with more content piled onto the old, it's about adding new content and game mechanics that work in synergy with existing content. Historically, Blizzard—and many other MMO developers—haven't been able to pull off the horizontal model for retail expansion packs. Players want a lot of bang for their buck, and the easiest way to offer that "bang" has usually been to make them more powerful by raising their power cap.

Some MMOs, like EverQuest, were able to pull it off, as well as those prized for their PvP, like Guild Wars, Ultima Online and Dark Age of Camelot. It's much harder to entice players to spend money on a horizontally-modeled expansion for an MMO that's built around its own hard cap and the PvE experience. Games with great PvP, on the other hand, lend themselves to horizontal expansion much better; players are willing to spend cash on an expansion when it offers new and fun ways to compete against each other, even if their "power" isn't technically increased.

While there's plenty of fun to be had with WoW's PvP, it isn't what defines the game. Whether it's the leveling experience or perpetually maxing out your character for harder endgame content, most people come to WoW for its PvE. But in the early years, Blizzard wasn't developing that PvE content in a way that could be easily adapted to horizontal expansion.

The developers began attempting some horizontal expansion by adding things like flying mounts, new classes and new professions to the game, but most of these changes came packaged with the traditional, vertical expansions. Blizzard recently started introducing more drastic changes between retail expansions, like the updated game mechanics and new content we've seen since the "2.4 era." This helped to retro-fit WoW into a game that might better support horizontal expansion, which is what we're about to experience in Cataclysm.

Although Cataclysm will raise the level cap like past expansions, Blizzard is only increasing that cap by five levels this time, instead of 10. This will be WoW's first retail expansion that isn't chiefly relying on the "more power" factor; in true horizontally-modeled spirit, Blizzard is changing the game instead of just piling on more new content.

As most fans already know, the entire Old World will be reshaped by Deathwing's emergence; almost every zone in the game will be physically and graphically changed to reflect the expansion's story. Aside from adding new character races, skills, a new profession and a reworked guild system to WoW, Cataclysm also introduces the ability to use flying mounts in Old Azeroth; a prime example of horizontal expansion.

In an earlier story, ZAM speculated about the impact that Cataclysm will have on WoW's long-term sustainability; we suggested it's as close to a sequel (or a "WoW v2.0") as we will ever get—and that it might prove to be the expansion that extends the game's lifespan by another five years. But Tobold and many other community members shared a similar notion more than a year ago, before Cataclysm was even a rumor. Is this a testament to the uncanny insight players have when it comes to their favorite MMOs, or a sign that there might actually be something to this whole "horizontal expansion" style of gameplay?