I know the "soul memory" (I think that's what it was called) system rubbed people the wrong way, since it was used for matchmaking despite including unused and permanently lost souls in its tally (so if you'd lost, say, a hundred thousand souls over the course of a playthrough, you'd be invaded by people who potentially had much higher stats than you who hadn't lost any to repeated deaths). Likewise it became hard to set a specific community-agreed-upon level for PvP due to the difficulty in stopping gaining soul memory (I think there was a ring that stopped you from getting more souls while wearing it, used for the purpose of locking a character in at a particular soul memory level).

I think there may have been some other stylistic changes to how the gameplay flowed too, but I only played DS2 and DS3 so I'm not sure what those changes were, I just recall hearing a long explanation of it all once but can't remember any clear details.