I love the fact that the developer can handpick components and easily customize them to their needs. I find this as a huge plus point of Semantic UI over other popular alternatives.

The default components are really easy to use and fit the fast developing norms

I love that there are so many elements to choose from, and each one has many different options for customization. I like how easy it is to implement and reuse from project to project. There are a lot of integration options, and the ability to theme custom variables is very useful to avoid having to redefine all of my styles in a project. I also think the documentation is pretty thorough, and I usually can find answers to what I'm looking for pretty easily. It also seems there is new content being added all the time. I like being able to add functionality that "just works" without having to worry about too many moving parts.