Handling a shitload of frequent rejection generally well, understanding your clients/customers' needs inside out and how your business fits into their solutions and generally being good at talking to/reading people, i.e. qualities that most engineers don't have. NOTE: People can be good at these things without being extroverted, unlike what many people think (and I thought until very recently)

It really comes down to making people (a) think that your shit (mostly YOU) will help them with their problems and (b) feel confident in trusting you.

As for comp: Most people in sales jobs go through ups and downs in their annual comp largely because most of it comes from commission and bonus. You could make >$1M one year and <$100k the next. Technical sales are more stable since salaries tend to be higher, but comp is still highly variable. Also, unlike engineering where one's performance criteria are somewhat superfluous, sales is a very numbers-driven job. Most have quotas they need to hit with bonuses for every level above that. Not hitting your quota == not having a job next month/year in some places.