For me, the people there are the main thing. Went to 2000trees this year and the vibe was amazing. Genuinely have nothing bad to say about anyone I saw there. The music is also a massive thing for me (I'm not into drugs and I only really casually drink, so the main thing I go to festivals for is the music). Finally, I think for a festival to be great it needs to have some sort of uniqueness to it. You can have a great line-up and set up but if everything is just generic, people won't feel like they need to come back as you'll have done nothing to stand out from the competition.