There are two different behaviors when assigning tags or types. Both scenarios below are based on using keyboard only (tested on OS X).
Types: Type the thought name and press the types button (or keyboard shortcut). Arrow down or type the tag name and hit enter to choose your type. At this point the cursor is placed back into the thought name. Press Enter again to Create the thought. Tags: Type the thought name and press the tags button (or use the keyboard shortcut). Arrow down or type the tag name and hit Enter to choose the tag. I think the hangup is here. The focus doesn't change back to the thought name after you press Enter because you can assign multiple tags to the same thought (or select the same tag you chose the first time to remove the tag from the thought. You can select a new tag to assign or, or press Esc to get back to the thought name and hit Enter to create it.