The line the cursor is on is now shown in the left sidebar as 0, counting up for each line away from the cursor. So now it tells you what number to use for "10j" and "10k" to move up and down a specific number of lines.
m[a-z]
`[a-z]
Mark a point in a file into a register, and jump back to that point. Or even more interesting:
m[A-Z]
`[A-Z]
Same as above, except capital marks are global and work across files.
Also,
``
Jump back to the last place you jumped from (but only within the same file).
My only problem with relative line numbers (and I use them despite this) is working with ranges of line numbers. For instance, I'll maybe be doing some refactoring and want to see what it looks like to move lines 127,137 to my current cursor (127,137t.). With relative line numbers I have to do some quick arithmetic to sort out the range.
Also,
Jump back to the last place you jumped from (but only within the same file).