I have played Vanilla since 1.1 or 1.2, so I guess I would still be considered a 'new' player in the game compared to a lot. I've gone through different mod packs, Darkness Falls, some Undead Legacy, Rebirth, but most of my time has been spent on Project Z.
I found that Vanilla lacked a lot of essentials to make the game long-lasting, basically once you hit tier 6 in weapons and armor, there really isn't much to do unless you are heavily into base building. When 2.0 came out I was somewhat excited about some of the changes, until I played it. I found that 2.0 was a huge step backwards, and the mod I was playing at the time, Darkness Falls, wasn't updated to the 2.0 release. Some friends started the new "Project Z" mod which was basically brand new at the time and we haven't looked back since.
After watching a lot of the scathing videos on how "bad" the game has become over the years, taking good parts out of the game and replacing them with things that may have made the game easier for a lot, but for people that liked the difficulty of the game it has been hacked apart to no end. I think if I would have stuck with Vanilla since about Alpha 16 I think it was, I probably would have quit the game long ago.
Project Z adds a lot of great features into the game, making it to end-game can take quite a bit of time but the rewards are great. The addition of boss mobs, although I think they pop up way too often, adds another layer of difficulty which a lot of games like this have, but vanilla does not. Even the addition of Elite mobs makes hoard nights, as well as regular gameplay, a lot more challenging and fun. Adding new workstations and ways to craft new items has been great. I do think that early on you get some of the good Project Z gear too easily, but getting to the point of crafting the top-tier items can take a very long time. I find this appealing because you are working towards something better instead of just getting your Tier 6 vanilla gear and restarting your server.