Features vs. Benefits
One thing I come across time and time again when reviewing onboarding experiences is the company trying to sell me on the features of their product instead of how it actually improves my life.
It's sometimes tricky to tell the two apart, but Jason Fried of 37Signals recently tweeted a great take:
"Here's what our product can do" and "Here's what you can do with our product" sound similar, but they are completely different approaches.
— Jason Fried (@jasonfried) November 13, 2013
That's still a bit hard to use as a litmus test for persuasive copy, though, so here's my own rule of thumb for telling the two apart:

People don't buy products; they buy better versions of themselves. When you're trying to win customers, are you listing the attributes of the flower or describing how awesome it is to throw fireballs?