First of all, it's "depth".

And Firefly isn't one of my favorite shows of all time, but I did enjoy it a ton. And I think if the show had continued, it had the promise to become one of my favorites.

The show is just flat out fun to watch. There are incredibly few adventure type shows to begin with on television and even fewer with the kind of wit and humor that Firefly had. Great dialogue, a unique premise, a wonderful cast that fit together brilliantly, these are the reasons why people fell in love with Firefly. Firefly had a special charm that's rare among TV shows. I don't even think Joss' other work had it. Buffy was funny, sarcastic, powerful and vulnerable, but not especially "charming". Firefly just had oodles of charm. And a big part of that I think was the rare way in which the characters kinda fell into place. Talk about a lovable rag-tag team.

If you don't get it, then you don't get it. It's kinda like trying to explain to someone who doesn't think Arrested Development is funny, how and why it's funny.