What is the hardest thing about salsa dancing?

This is a very subjective question, it depends on each dancer's perspective but there are few basic things that most of the salsa dancers must be facing it.

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 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.

I can tell you that it takes around three to four months to get to basic level of salsa. If you attend classes twice a week and practice on your own another two hours a week you will be able to dance the basics of salsa soon. The learning gets harder and takes more time after the beginner level though.