Power progression means getting stronger, progression simply means obtaining more for your character, more to do. Collecting is a form of progression, for example. Getting alternative drops of the same ilvl that you already have is horizontal progression. Generally, it does refer to cosmetics, since we're talking about shifting power around and generally keeping it at the same level, as opposed to getting a direct 'ilvl upgrade'.
It wouldn't only be visual, as glyphs originally were able to change the function of abilities completely. They are basically like choice nodes for all your existing core class abilities.Ok yeah, so you are talking mostly about customization - which would be better implemented as part of the Barbershop, letting you choose between several visual variations of a spell, be it base color/tint, or changing a lightning-based Thunder Clap into an earthen or voidy appearance.
The talent system is more prone to cookie cutter because it is limited by choice options, and all talents are progressed vertically. Power is applied by branching down new nodes that are more powerful than anything that comes before it. But the branching doesn't give you much options and everyone ends up going down generally the same path because that'a how vertical progession ends up being designed; tou often get shoehorned because everyone is incentivized to put points further down the tree rather than 'hybrid' picks in a different branch up top. This limits the overall possible combos, because no one would sacrifice power for hybrid utility.
Horizontal progression if your talent tree spread out and grew sideways, giving you more options per existing choice node without affecting your overall power.
So an example is if the old pick-out-of-3 talent system was expanded horizontally, you would gain more options over time by making it pick-out-of-4 or pick-out-of-5 and grow it sideways. And to add on this, you aren't just limited to one talent pick, you could mix and match and apply multiple glyphs to the same ability, so you could get a blue colored fireball that slows and AOE's while trading off a lower crit rate or increase cast time or whatever works to keep it all relatively balanced. Each glyph could carry its own set of checks and balances.
Of course, I'm purely theorycrafting here, so I don't have the design experience to back up this working at all. Maybe the devs did try this, and it was just too much hassle for the sake of customization. We won't know for sure since so little was shown with the original Glyphs.
The glyph system in Wrath-Cata were not very choice driven and the limited options again forced most players onto the same set of 3 Major Glyphs. If you're a tank, you picked the 3 best tank Major Glyphs and that's it. You aren't given room to pick the others because it competes for a Major Glyph slot. An ideal Glyph system would work more like Transmogs, where you apply it per item, not all competing for the same 3 slots.