Don't get me wrong. I like Salesforce Lightning Experience. I LIKE it, but I don't LOVE it...yet. The interface for Lightning Experience is definitely better, and more modern than Salesforce Classic. And I've noticed much needed speed improvements coming to Lightning. Lightning's performance is almost living up to its name, finally. What is holding me back from fully embracing Lightning is the sheer volume of functions that are still not available in Lightning.
This lack of functionality in Lightning leads administrators and developers to have to continually switch back and forth between Lightning and Classic. I wouldn't encounter these gaps if I wasn't going all over the platform creating training courses. Let's just say that more than a few times I've encountered dead-ends with Lightning while recording lectures. All I can do is instruct students to switch to Classic to perform certain functions. With that said, Salesforce has been steadily chipping away at their very long list of gaps between Lightning and Classic.