I was usually the DM for my friends...
1: Because even back then I was the best writer in my circle of friends and
2: They loved my style of "The DM Giveth, and the DM Taketh Away."
There was a reoccurring character in my campaigns, some unknown deity that was fully capable of shapeshifting, but for whatever reason always prefered to appear in the form of a happy little fluffy white baby seal. Now the first time he appeared, he showed how truly powerful he was by vaporizing the first fool that tried to attack him. The player was reincarnated as a lilliputian and remained that way until the end of the campaign, at which point he returned to his normal size.
Future attempts to attack the seal were met with insta-deaths...but that's because you're not supposed to attack him. You're supposed to let him flop around and make cute noises and enjoy himself as he does random things. Sooner or later he'll cough up some kind of 4th-wall-breakingly named loot that was quite good (Sword of Infinite Buttsecks, for example...I forget the stats I put on it but it was quite game-breakingly ridiculous).
What's the flip-side of such generosity? Well I apply to Tyco's (Penny-Arcade) brand of DM deviousness...inescapable traps and situations that lead to insta-deaths.
For instance, our heroes hear a rumor about some ancient temple deep inside a cave that houses a magical chalice that will grant immortality if you drink from it, but just like the Holy Grail from Indiana Jones, it cannot be removed from the temple. Lured in by the prospect of such an artifact, the party sought out the cave. After fighting their way through hordes of zombies, vampires, and skeletons, the party arrived at the temple and were quite surprised to find that the temple itself was utterly devoid of monsters. This should have been their first hint that this was a baaaaaaad idea. But greed for life unending pushed them to the central chamber where, on a dais bathed in a golden light, rested the legendary chalice. The ancient elven script on the pedestal read simply "Any that drink from this chalice shall have existence unending." Fair enough, they filled the chalice with water from a nearby fountain and they all took a sip. A few moments later, they were all turned to ash.
You see, the immortality that the chalice grants is to turn the drinker into an undead...the temple the chalice was housed in had been sanctified and consecrated by a powerful holy aura...an aura powerful enough to reduce any undead caught within it to a pile of ash. This is why the vampires, zombies, and skeletons in the cave were not also in the temple.
>:3 yeah...that one kinda pissed them off.