In the "vertical" progression you have unlimited skill slots, but in order to make skills usefull you need to invest a limited ressource in them, mostly skill points.
In the "horizontal" Progression you can generally get every skill (or almost every skill) and they will always be at maximum effectiveness. However, you can only use a limited number of skills due to limited slots.
In the past, most RPGs would use a vertical progression, where higher level and better gear where what mattered to make skills powerfull. This has worked incredibly well in say DIablo 2 or WoW.
Nowadays, horizontal progression seems to be the new trend, with DIablo 3 and Guild Wars 2 for example going for this system. The idea seems to be that this reduced the need for any "grinding" to get good skills and instead focuses on the players ability to select and correctly use the skills. To this end, the skills are usually significantly less powerfull individually than in a vertical progression and have to be combined for good effect.
While a "vertical" progression works by the player deciding for a build that, in his opinion, can master all the situations he wants to get involved in, and then sticking to it, a "horizontal" progression works by the player deciding what build is best for the next task.
Technically, this should give players flexibility to play however they want. However, I feel like that idea is somewhat flawed: It assumes that during normal play of an MMORPG, Players will know what situation is coming up ahead, and prepare their skills accordingly. But that implies that you already know what you are facing, which usually only happens when you grind the same encounter numerous times or when you are playing PVP.
During levelling, you cannot really switch skills based on encounters, because you are already IN the encounter when it happens. So you need to select a skill build that gives you maximum flexibility. However, horizontal progression systems usually do not give you such a build, because that would eliminate the need for any further progression.
The result is a feeling that none of your skills are really "good" for what you are doing, which seems to be voiced by quite a lot of players regarding Utility spells. Personally, I miss the feeling of looking forward to that one spell that would drastically change my game e.g. in Diablo 2.
In PVP, Vertical progression systems can be designed to require player ablity (Just look at MOBA games, most have very traditional, although short, vertical progressions), and hence horizontal progression looses a major selling point.
tl:dr: I think that the Idea of swapable Utility spells instead of a more traditional skill point system does not work well for MMORPGs without a complete PVP focus, i.e. with a traditional PVE leveling system. Horizontal Progression only works well in games without a level system or some kind of non-traditional system. Since Guild Wars 2 already uses a different form of Character for PVP anyways, why not combine a vertical PVE progression with a horizontal PVP one?
Edited by Cronos988, 05 December 2012 - 09:06 AM.