Coming in a point of view from someone who has never donated and has played for 5+ years.
Customs are OP, whatever balance was attempted.
What made me want to quit?
1. Customs - see above
2. OP bugs - lots of it, but the worst one was the talent stacking. I believe lots of people knew it (maybe a few before me) but I am pretty sure I was the first one that "perfected" it. That is stacking 142 talent points.
I am personally guilty of exploiting and enjoying it for few months before reporting and even demonstrating the steps to Bobby. 1 year forward, it was never fixed and everyone is doing it already.
Real reason that made me quit?
1. Player base - w/ custom or not, what's the purpose of playing if there's no one to play with?
There are these left and right reasons why I wanted to quit, but never did until player base sank to 2 digits.
Now you might wonder why NE had high player base even with the customs, personally I think it's because:
1. Custom donors may be OP but still tolerable - just avoid PVPing with them.
2. There's no other stable server until THAT server opened.
What happens when another server opens that solves those two issues? Player base slowly but steadily sank - server dead.
Again, this is just a non-donor point of view that hopes this post could help the revival of NE.