As taken from this article at
http://massively.joystiq.com/2014/02...l-progression/
Horizontal progression
Horizontal game mechanics make players think outside of the confines of linear progression as the games that employ them offer elements that actively cater to individualistic strategy building. Rather than equipping characters to face the latest content the MMO world can throw at them, horizontal mechanics point towards the development of a wide range of character attributes that will weather any challenges that lie ahead. Each ability isn't inherently better than another, but well-researched combinations might create a much more powerful effect. The key here is to expand the pool of items, abilities, and systems that are useful and interesting during the endgame.
Horizontal mechanics, normally equated with sandbox MMOs, faced a decline when WoW's phenomenal success caused an explosion of clones to wash across the genre. The vertical progression mechanics offered by Blizzard presented players with a popular lower barrier to entry and the company's competitors adopted a formulaic approach to themepark MMO creation. However, vertical progression can also create an all-too linear experience that makes progress feel compulsory and one-dimensional. The choices presented to players don't ultimately matter, and characters will be equipped with whichever items best boost their key stats.