Vbulletin uses a unique tagging system that allows users to add tags to threads so they can be sorted and you can see which are most popular. It is not obligated of member's to post tags, but it can help out. However, the key is not to overdo it:
Common Tags
Certain tags are just too common to be added. Otherwise, they clog up the tag cloud. The numero uno example of this is "Pokémon" (or "Pokémon", etc). Prior to today, this tag caused all other tags on the tag board to be rated either 1 or 2 (of 5). This is not good. This is a Pokémon forum, after all. Other examples are general emotions, like "funny", or small words, like "hot".
Redundancy
It is not uncommon for a tag to already exist. Prior to today, "Pokémon Black" and "Black" both existed - they now are both just "Pokémon Black". Never post redundant tags like this, and always try to mimic any existing tags.
Spam
Tags must mean something. Currently, spam tags exist for "ahhhhhhhhhh", "mudkipz", "aries 26 arf hi im new" (seriously?), "budist" and "buddist" (do you mean Buddhist?), "whyareyoureadingthetags?", ...you get the idea.
Spelling
Don't be cute with your spelling - if you need to be cute, do it in the title. So "ever" instead of "evar", "Pokémon" instead of "pokemonz", etc.
1 – Cat333Pokémon