@Kazamas-Keyblade:

A lot of things. But Id have to say the main ones are:

1. The brokenness of the characters - most of the characters have gone through some sort of trauma or another, and you can tell in everything, the way the act, talk, it's in their very soul, making the characters, who normally would be considered annoying, very relatable and very realistic.

2. The religious undertones - the creators themselves said they had nothing really in mind when they used a bunch of religious (mostly Christian) references, but that doesn't mean you can't read into it. It can get super deep.

3. The philosophy - there are a lot, and I mean a lot, of deep philosophical questions asked throughout the series. A lot of questions I've never thought of before, a lot of abstract ideas and meanings that each person will interpret differently. Some of the questions they ask are deeper than the ocean. Not kidding.

And that's what makes Evangelion good - or, at least, what I think makes Evangelion good. Different people may have different opinions.

@KyraChan:

Something had to make it as famous as it is now.
But what about it was good?

@Kazamas-Keyblade:

You heard right. Evangelion is flawed, but those flaws somehow make it more... endearing, at least in my opinion. Except for the final two episodes. As episodes, they're... okay, having both extremely good and extremely bad qualities, but as a quote-unquote "ending", they're terrible. But End of Evangelion fixes that. Mostly.

I've never seen Evangeleon but from what I've heard its both good and bad.
(Plus I heard Michael Bay was speculated to have been given a shot at doing the next Live Action movie)