If Richard has a "fatal flaw," this might be it. The play is all about Richard's reluctance to actually do anything when he's directly challenged:
Richard is not what you would call a man of action. Henry Bolingbroke, on the other hand, is all about aggressive action. When he sees a chance to get the upper hand or gain power, he takes it. Shakespeare is asking us to think about whether a willingness to take action is what makes for a good king.
Although the play starts off dealing with the aftermath of an active crime – Gloucester's murder – Richard's real failure is his inability to act.
Richard's passivity might have been bad when he was king, but his ability to talk about what he's lost makes his situation pretty moving once he's lost his power.