The most important thing when it comes to RPGs, is that they gotta be fun. I won't try to define what fun is, but I'll share some of my group's fun moments, and some not so fun ones as well.
It was FUN:
- Capturing or charming goblins and kobolds and having them become well-loved NPCs. Once the party wanted to sneak a kobold they recruited (named Skrittles) into town. They had to dress him up like a gnome so the guards wouldn't kill him. I pulled up a picture of E.T. where he was dressed up like a girl and we all laughed our asses off.
- Giving the PCs a home base and letting them load it up with cool shit. As long as the players aren't braindead, this is always a good time. Fire poles, waterslides, beer taps in every room, the sky is the limit!
- For me, I always love to put the party in the middle of a moral dilemma. Not sure how fun it is for the players, but I sure love making them second-guess their actions. Humanoid babies are great for this. Another great one was having the party encounter an imprisoned devil that had something that they didn't need at all, but they REALLY wanted (i.e. overpowered magic item). Watching the players debate what to do for nearly 30 minutes was priceless in my book. Of course, they let him go in exchange for the item, and of course it turned out to be an evil-aligned intelligent weapon. Man, being the evil DM can be fun sometimes!
- Pets and familiars are always fun. A player in one of my games saw chickens for dirt cheap on the equipment list, so he started the game with 50 chickens and referred to them as "the council of feathers" and consulted with them often in play. An unfortunate encounter with an unhinged dungeon door wiped them all out, but it was fun while it lasted.
- Letting a player play a doppleganger, killing him (not on purpose of course), and have the doppleganger become an undead NPC, bent on revenge since the party decided to just leave his body there in the field to rot. The doppleganger then takes the form of some sexy maiden the PCs are hired to rescue a few sessions down the road, and when the moment is just right, he reveals his true form, and the corpse of the girl they are supposed to rescue! This was an all-time favorite of mine, and it happened in a completely organic way. Couldn't have planned for something that cool if I tried.
- Random mutations are something my group seems to love. When I presented my half-fiend race with a d12 table for the random demonic features, 3 of 4 players rolled one up.
- An occasional laser gun from an age long past hidden in some rubble or dirt. You never know how many charges it has, or if it will backfire in some terrible way!
- Random tables for crits, fumbles, and just about anything else. I don't know what it is, but every time a random table comes out, hilarity ensues.
- Having the party shit all over your carefully designed plot. This used to drive me nuts, but since I adjusted my attitude and take it as a given that this is what they are going to do, I can have a lot more fun improvising.
Not so fun:
- I once implemented a three critical misses = broken weapon rule without warning my players beforehand. I'll never do that again. What I thought would be an interesting challenge was perceived as me fucking with them in the middle of a combat.
- I used to have a tendency to talk with the players a lot after a game session about my designs and things I expected them to do versus things they did and did not do. After a while I learned it's better for the game to just leave these things a mystery to the players.
- Playing Craft(Alchemy) by the book. It lets you make one or two items per game week basically, and you still have to pay 1/3 of the retail price for materials. BORING! If I had to do it again I'd just let them make whatever they can carry, maybe at 25% cost or something. It's not like tanglefoot bags and acid break the game (at least I don't think they do).
- Skill systems in general. Maybe it's just my ineptitude at running them, but it always feels clunky, and they always fail the search check when it's most important for them to find the thing, whatever it may be. I've since gone to a more narrative handling of skills, with them telling me what they are doing, and me providing them with the logical result of their actions, rather than letting dice decide these things, and it works much better for us.
- Identifying magic items. I haven't found a good method for dealing with this, so I usually just tell them what it is (which feels lame). I'm open to tips as to how people do this, particularly with regard to the bookkeeping aspect of "so-and-so found this thing and it does this and this, but they don't know that yet". What a pain in the ass!
- Letting the PCs play monster races, but then not letting them into town due to the human's hatred of them. I was going for the bane of all games, "realism", but what I got was a big batch of shitty un-fun gaming.
So, in looking this over, it certainly isn't the most exciting reading. I think a lot of it falls under the "you had to be there category," but hopefully someone will pick up on one or two things that were fun or unfun for my group, and not have to learn the hard way like I did. Happy gaming!