In the first age, pretty much anybody who wanted to be an immortal had a shot, as long as they weren't tagged as cheaters. Most of them washed out around 52-53, but if you put in a lot of hours you got promoted. There wasn't a formal process or any expectations set in stone, but it was generally accepted that you should write an area or make a significant contribution in coding.
I stayed an imm for around 6 years before getting tired and deciding to take a break around 1999, after a few run-ins with Pico and Cador. There were no accusations of cheating or anything, It just became a chore to login after awhile. To keep reading the same descriptions, having the same conversations with empowerees and cabal applicants, catching the same people cheating... it just wasn't fun anymore.
I kept regretting giving up a level 59 char though, and kept thinking I should make a comeback. I tried to re-imm 3 more times between 2003 and 2008. The last time they let me skip the area writing process and just bumped me to 53 because they wanted me back. I still burned out though. I will say the heroimm-promotion process became much more clear and achievable each time, but there is still an enormous amount of time, effort, creativity, and especially commitment needed for long-term and often mundane tasks. There is a *lot* of time you need to spend doing things that simply aren't fun before you are trusted to do your own thing. That is part of the weeding out process, as you need to sacrifice your time to show you won't abuse the power when you get it. And there is a lot of power and trust at stake at the higher levels.
I have written 2 areas that are still in the game, but I also have around 4 unfinished or unpublished areas in various states of unreadiness. Designing and writing the area is easy, it's the hammering out of all the small details and testing it that kill your enthusiasm.
There was a huge difference in my mindset when I was 22 and had nothing but time to sink in, and being 32 and having kids and a full time job and other responsibilities. I wanted the old imm powers and comraderies back, but I didn't have the time or spark needed to make it happen.
I still don't, and don't think I ever will. But I'm glad to see others do, and they keep the game running for the odd times I feel like logging in.