Most of cheating is botting. When bots farm dungeons or other activities, earn gold and then that gold is being sold at black markets for dollars to other players.

That's indirectly hindering other players progression, because it causes deflation (so you can't earn as much gold selling your ores); because it causes inflation (more circulating gold, yes, these are contradictory); because it denies other player farm (if bot gathered ore, other player have to search for another vein) and so on; also illegal gold selling increases expectations (other players bought super good gear, why don't you do that) and causes burn-out (because farming gold fairly is much more hard, than just buying it).

But mainly it just makes players angry, because they can see these bots moving in a predetermined route and stealing resources from their noses. I'm not really sure if bots are that bad in the grand scheme of things, but living players certainly don't like to compete with automatons.

There were also cheaters who used instant cast interruptions at arenas, but it seems that competitive PvP is not that popular nowadays so I'm not sure how it's wide spread.