What I consider essential, the sort of things that are make-or-break:
Gender preferences?
How often do you post?
Plot vs Smut ratio? (If someoen is looking for 80 plot/20 smut, that's really helpful to know that)
Favorite kinks and top limits. [i}Even if[/i] you are very flexible and are okay with a lot, knowing what you enjoy the most is helpful.
Where do you prefer to RP: Threads? PMs? Discord?
Dom/sub/switch/not at all?
Next are the Important stuff
Anything you are Specifically looking for.
Genre: What sort of setting or tone are you looking for?
Plots: Always helpful to have something as a spring off point.
Pairings vs not pairings is a definite argument. I find them useful not as specifics, but it gives me an idea of what the person is looking for. For instance if I see "Boss x secretary, teacher x student, doctor x patient, coach x player' I instantly know this person REALLY likes power dynamics and potentially abuse of authority. If the person hasn't listed their favorite genres, it shows where their mind is--fantasy vs modern vs sci fi.
Finally there's the "not necessary but helpful"
Themes: these are the tropes that you find yourself gravitating to/really enjoying. "Vampres" "Arranged marriage" "Beauty and the Beast" "First love" "Bratty characters" "Muscled bad boys".
Writing sample (If you RP in threads I think a sample isn't as necessary)
Face claims: Real, drawn, or any at all?