To add to Arquenya's post, one important thing about horizontal games is that they are kept balanced by limiting the number of choices available at any one time. For example, GW1 had an 8-slot skill bar. In MtG, this is done in another way: your deck needs to be at least 60 cards and have at most 4 cards of each kind (except certain cards). You are however allowed to have as big of a deck as you want... But you don't want to go far above 60, because that makes your deck less predictable and that in turn makes it harder to control it and play your strategy.
How far should horizontal progression go then? I understand that people want horizontal progression because it doesn't take them any work to get results other than learning the basics and just a bit more. How much of horizontal progression is enough? GW1 had so much different skills added throughout it's lifetime that it became convoluted and some of the skills were useless and/or outdated. Besides of skill utility increase, what other horizontal progression could there be added? New kind of problems most likely, but how much of that can be kept up without rehashing old mechanics? One of the good points of horizontal progression would be that all new and old content would be relevant and coming in to do anything would require no prerequisites. I think a game going for pure horizontal progression would require some serious innovation and implementation that no one has even given thought about yet. It sounds great on paper and as an ideal, but getting it to work on an MMO that's supposed to draw millions of players in is not very likely.
GW1 spun out of control in a way, because of power creep and because the number of combinations were too many to balance. The MtG method is in a way self-balancing, and that is a very important concept to horizontal design. Another example of self-balancing mechanics would be some of the most well balanced skills in GW1:
Frenzy: Probably the best general IAS in the game, but one that viciously punished you it if you use it at the wrong moment. Not only does this self-balance against the user, but also against the opponents, who are (doubly) rewarded for keeping a lookout for people using Frenzy.
Reversal of Fortune: Like any prot, this one is wasted if cast on a character that doesn't come under attack. But even more so in this case, because the skill has a potentially massive heal that triggers only if you cast it well.
Diversion: The skill that pretty much defines much of what mesmers were in GW1, it yields a massive reward if used right and is just a waste of its quite long casting time if used wrong.
In all of these cases, the actual numbers were not what kept these skills balanced, it was the mechanics behind them that did it. Of course, a 100% IAS in Frenzy instead of 33% would be unbalanced, but removing the double damage would be a much worse hit against the skill.
Why am I blabbing on about skill balancing when the question was how much horizontal progression there should be? Aha, because it's intimately connected. There must be self-balancing mechanics involved. There must be tradeoffs. Tradeoffs that feed back into the use of the skill, the construction of the deck, etc. If you run out of tradeoffs, or if they don't work, you should stop putting out mechanics. In other words, there can be as much horizontal progression as the mechanics allow, and there can be as much mechanics as the tradeoffs allow.