• It's my first time seeing the page, I want to know what it is. I definitely don't want to load a config. The default config should be the first thing the user sees, as it does a reasonable job of explaining the concepts.
• The button for adding a card is tiny and far away for where the cards get added.
• The menu being hidden by default means its items are not at all discoverable. It could still be collapsible, but open by default if you want users to actually use it.
• Pressing ESC in the initial modal ends up in an irreversibly (without refresh) broken state.
• The only way to save appears to be exporting a JSON file. You should look into LocalStorage or IndexedDB as a way to autosave so the user doesn't lose their content by inadvertently closing the tab, navigating or refreshing.
• Deleting a column is instant and irreversible. You should have confirmation, or, even better, undo.
I'm guessing you've built this to fulfil a personal niche, so kudos for getting it built. God knows I never finish anything I build for myself.
[1] http://pragprog.com/book/lmuse/designed-for-use (or if you google hard enough there are ahem totally legit free PDFs)