Classic features missing in LEX
Though Lightning has been used for over two years now, it is still incomplete and new features are added to it constantly. Right now there are some things that, according to some users, are better implemented in Classic UI than in Lightning Experience. Here are the most prominent of them in our opinion.
Probably the biggest benefit of Classic over Lightning is its higher customizability. Classic UI allows each user to add tools and apps to the utility bar and personalize the home page to their liking, which isn't available in Lightning. With LEX, it's up to admins to decide which apps and in what order are displayed in the utility bar. Still, users can add tabs they use frequently to their favorites.
Another feature that Lightning users are going to miss is some functionality regarding Opportunities and Leads. For example, Opportunity Teams, Splits, and Big Deal notifications are only available in the Classic UI, and so are lead campaign history, change lead owner and duplicate lead merging features. To make up for this, Lightning Experience has a customizable highlights panel, activity timeline, sales path feature and an opportunity board which we've already mentioned.