Creative use of game mechanics

Cancel logout macro allowing you to reach stupid heights of exploration.

Clever use of game mechanics was a term used to describe exploits that were usually in the grey area of being usable or a bannable offense.
For the most part these were mechanics that didn't have too much of a direct impact to most players. If it impacted the economy heavily, or let someone solo a current tier final boss then it would be considered a game breaking bug. Generally these days the only real clever use of game mechanics are raid/dungeon/m+ based to force an upper hand on a boss and it's blizzards job to patch a fix as soon as they can. There were a large array of these in dungeons which people used heavily for increasing their speed throughout M+.

Severe Exploits

• Changing game files on the client so that WSG doors or BWL doors were not visible on your client allowing you to walk right through.
• An exploit using a macro where you could jump logout and cancel logout which would let you float in the air and gain height resetting your jump. It allowed you to gain extreme height and was very quickly fixed.
  
• Breaking Bijou's for Zandalari reputation when your bags were too big to receive the token it created. This could be used to cap out reputation quickly. It can also be used to get your account banned forever if you're Popp.

Neat tricks

• Out of combat resing in raid encounters during early vanilla before they forced boss encounters to make a zone wide combat flag.
• Force Hearthing was a way to use a hearthstone when your one is already on cooldown. You would invite someone to a party while inside an instance, leave the group so the instance then becomes the new persons instance forcing you out after a 60 seconds timer. Back in Vanilla to Wrath hearthstones were a 60 minute cooldown making this strategy very useful to travel.
• Disgusting Oozling pushing debuffs off. Mow used this to remove trick or treat debuff allowing him to steal all the candy from kids. Shadowalker used this trick to push the stomach acid debuff off during the C'thun encounter, making the fight significantly easier.
• Mind Vision used just before a stealth class like rogue/druid would stay on them even while stealthed so you could just Shadow Word: Pain them out. Didn't get fixed until early MoP.
• There was a console script command which let you turn upwards until you were looking straight up, but instead of that being the limit, it let you continue going upside down.

M+ strategies

• Vault of the wardens
  • The last boss of this instance required a light to show various things like unhide the boss which would come steal your light source and show the holes in the platform wide wall for the safe area's. There was a way to bring a second light into the fight which was intended. You could throw a light at a specific spot going down the stairs before you hit a trigger which would remove it, run past that trigger and pick it back up. This allowed you to have 2 lights for the boss speeding the kill up ever so slightly.