The best thing about this app for me, or any other tool I've used so far (evernote), was that it made writing my blog posts/blogging easier than with just plain notepad++! It's easy to use too - no need of command line knowledge which makes life much simpler when you're using stackedit as part time job like myself :) There are several plugins available via their website but nothing really special there either except maybe one called "marker" plugin where we can highlight blocks while editing our post etc., i don't remember if they have more now :).
Maybe some people will find its UI design looks outdated compared others though? If anyone uses these tools then make sure your favorite IDE integrates well into them otherwise all those features might be wasted because u'll end up doing everything manually from scratch again ;) Also try out different themes / colorschemes provided by default;-).