there's honestly a lot of things that could be considered most important in a roleplay. like communication, as already stated.
in a roleplay, there's you, then there's your fellow rpers. for you, many things are important. alongside communication with others, you need to maintain passion. it's never fun when you get into something and it's just a phase. next week you have no motivation for the rp in question because you've moved on to something more appealing. more often than not, this kills a roleplay. do what you're passionate about. it makes for more engaging and longer lasting roleplays.