Tags would be much too unwieldy for this. Keep it very simple. How about a short-list of categories selected from a drop-down? This would need more thought but

Tags would be much too unwieldy for this. Keep it very simple. How about a short-list of categories selected from a drop-down? This would need more thought but a very short-list like: 'falling', 'lucid', 'nightmare', 'premonition', 'embarassing', 'funny' etc. Nothing more is needed and it makes browsing by category fun for the random surfer.