It's certainly not trials specifically. It could be argued that pvp in general results in nerfs having an effect on pve. But seriously, has any nerf or change made pve less fun to play? There are so many weapons and abilities to choose from there is always something fun to use in pve.
Weapon performance and metas completely revolve around what the competitive section of the community is using and complaining about. The most prominent competitive aspect of Destiny is Trials by far. Trials pushed people into trying harder and abusing the meta. Most people claim that year 1 was the most fun, and that's because it was true. Trials came out with HoW an Crucible has never been the same since.