This is a very subjective question; it depends on each dancer's perspective; however, there are a few basic challenge that most salsa dancers must facing.
In Salsa dancing there are two major areas of focus one is Social Dancing and other is Stage based which includes performances & competition. Each area has its own challenges and areas of difficulty. I have summarised below with respect to social dancing:
It's one thing to do salsa patterns irrespective of the music and basic salsa timing & rhythm, this is what most beginners do, keep doing the pattern without necessarily even being correct on time. Then the next stage to conquer the salsa timing, rhythm & tempo. Once we have understood this we move on further into exploring Salsa musicality, this is where the real challenge and fun begins, understanding the deeper nuances of the music & instruments. So now it's no more about doing salsa patterns or partner work it's much more than that.
To be musical salsa dancer is the real challenge in every aspect. Of course for that one needs to start studying the music and also start feeling the instruments and executing that into your dance, like doing a solo sequence or shines sequence creating impromptu shines, especially of music which had not heard of before. When doing musical partner work, it's a challenge to interpret the finer nuances and then you have to also make sure that your follower understands your lead and is capable enough to execute it.
So one needs to work on many aspects at the same time, music, technicality, body posture leads and follow and making a beautiful amalgamation of everything is the real challenge and one of the hardest things.
PS: Taken from my original post on Quora