You're saying is that your product strikes a middle ground, "providing powerful project management features without the learning curve."

Why is no learning curve important? Because it allows teams to transition to it without taking time to do it. Teams that are already using complex products like Atlassian, etc. have already invested a lot of time into getting their whole team over the big learning curve, so they don't really have any reason to go with you. They would see it as a negative acknowledgement that they wasted time learning Atlassian, and since Atlassian already has more features than yours, they would be losing capabilities too (remember they're already over the learning curve). These are therefore the people that will say they don't understand your USP. You're not going to convince any of them to transition to your product.

The main persona you should be selling to is the people that either currently use the less complex solutions (Trello, etc.), and also the people that are just starting their first product and are just starting to look at the tools available (i.e. they haven't been exposed to anything yet). These are the people that will immediately understand your USP, and it will resonate with them. They want better tools without the learning curve. However, aside from time to learn, another reason some of these people have not gone to a more complex solution is because they cost more. So to get those people too, make sure you have a long free trial period to let them get a feel with their own experience that there's no learning curve, and to get them hooked on the advanced features, so when they try to go back to Trello, etc. they get frustrated with the annoying repetitive tasks, etc.

If you'd like suggestions more tailored to your specific product, send me a link and I'll check it out,

Lee