Glyphs - Customization based on the situation.
Prior to 4.0.1, I'm sure most of us purchased the six best glyphs for each of our specs.
We learnt them, and then forgot about them.
The cost of a glyph might have been 25-50g, if it was a more rare glyph that was available less (supply/demand) because it was a top choice for your class/spec in their chosen element of the game PvE/PvP.
Now our glyphs cost so much.
But it is a one time expense; once you learn it, you have it forever.
And the cost of Vanishing Powder is almost nothing.
So we can unlearn a glyph for a very small cost, and replace it with an already known glyph for no cost beyond learning it the first time.
This opens up a new tactic.
Unlearn Glyph A, which we use in Situation A.
And replace it quickly and easily with Glyph B, which is better in Situation B.
Do the equivalent of 5-min Fire Elementals in Battlegrounds, but switch to Lavaburst or Shocking in Arena.
Possibly even... given a bit of time, changing the glyph mid-fight, after seeing what the opposition is fielding.