"More" is relative: A is more important than B, but C might be of greater importance than A or B. "Most" is absolute: if D is the most important, it's more important than A, B, or C. It's important to turn the lights off before you leave the house. But it's more important not to leave the water running after you fill the bathtub. Most important of all is never to light your house on fire.

The difference here is between "most" and "more". You can use "more when comparing two things, and "most" when comparing three things or more... However, "most importantly" can be correctly used even if you are talking about only two features of a product for example to emphasize the importance of a certain feature.