Borrowed Power: progression systems designed to be used for a single expansion, then replaced some ways into the next expansion by a new progression.
Borrowed Power isn’t just things like artifact weapons, essences and covenant abilities however. In the past, we had forms of borrowed power that were much better received by the players - tier set bonuses, and glyphs.
Explanation of these in the spoiler
When people think of borrowed power, they usually think of the flashy examples from recent expansions, like artifact weapons and essences. But borrowed power existed in much older expansions too - the most common forms for it were tier set bonuses, and (back when they impacted your power) Prime and Major glyphs.
Tier set bonuses are the oldest form of borrowed power. Most of them change how your character plays, and all of them are designed so that they are good for the expansion the set is found in, but get replaced in the next one - either willingly by the player as the gear becomes useless, or forcibly by Blizzard if the set bonuses themselves are disabled server-side (immediately, or when you hit a certain level.)
As for glyphs - back when they existed, Prime and Major glyphs were designed such that you needed to replace the past expansion’s glyphs with those from the current one, even if they did the same thing. The basic reason for this was so that you had to use the newest expansion’s herbs - supporting the market for those professions. That design requirement to upgrade or replace your glyphs in the next expansion made glyphs another form of borrowed power.
Given that we’ve had these expansion specific progression systems for almost the entirety of WoW, why is borrowed power seen so negatively now? (I do actually have my own ideas on reasons why, but I was curious with what the community thinks!)