Important Elements of a Bullet Journal
These are all my opinion. The whole point of the Bullet Journalling system is to be flexible and customizable to the individual's needs.
1. Analog Medium
As in paper. The last thing you need is to spend more time on a screen, in spite of the fact that it may be tempting as a way to organize and plan.
2. Thick paper
If you write on every page of a notebook and create "spreads" with drawings, patterns, lines etc. you don't want the ink from the previous page to "bleed through" onto your current page.
3. Numbered pages
This isn't crucial, since you can number pages yourself, but one less repetitive task is always welcome. I don't want dates on pages as a given page may be dedicated to something besides a daily log entry.
4. Non-blank pages
A dot grid is preferred, followed by a lined grid, followed by horizontal lines. I currently have a blank notebook and it's very difficult to organize the space into anything neat, never mind meaningful.
5. Book marks
A little ribbon attached is a nice-to-have
6. Opens Flat
This is very handy for making sure you can access and write on the entirety of the page, or even create spreads that straddle 2 pages.
7. Hard Cover
I throw my journals in a bag often, and having them be bend/fold resistant is important, otherwise, they'd be ruined.
8. Extra Accessories
A pen loop, a back pocket for loose notes, stickers, and even little rulers to help divide the space within the page are available on the official Bullet Journal.
9. A5 Size
I think this is the sweet spot between portability and having enough space to contain a useful amount of information per page.
10. Between 200 and 300 pages
I think this number of pages means the journal can capture a few months' worth of your life, without becoming too cumbersome. After a while, you look forward to starting a new notebook style and playing with something new.