I have an achievement permanently stuck at 94%, because I ran out of time to gather influence before Heart of Thorns launched. This means I nearly always have an irritating, impossible task taking up one of my three preview slots for nearly-complete achievements. I am told by WvW players that many achievements there also tend to clog up that otherwise useful UI feature for a really long time.
The suggestion: a toggle in the nearly-complete achievements window of the Achievement Panel that lets you prevent a certain achievement from being displayed there. Kind of an opposite of the Watch function, if you will.