Slowly, the one that drives me nuts the most is changes to the rules system. It never, ever seems to work out when the DM introduces setting specific major changes. I love story and so I find it kind of cool as an idea at first.
But it can radically change the balance of a game and make things a lot less fun.
In particular:
1) Mass banning of certain spells or classes of spells in order to enforce the campaign theme. This can go very badly, very fast. For example, no teleport spell seems like a reasonable choice. But then you decide it's a water themed world so only water or ice damage spells exist. And then you note that summons don't work including things like the mount spell.
After a while it can really effect playability.
2) Very restricted character classes -- a party can be awkward to balance if you discover no healing classes are allowed and so forth.
3) Restricted items and technology. Wizards can't get spellbooks (I watched this for 8 months once). Magic items are nearly non-existent. Standard weapons are not available to be purchased or used (okay, I can understand the spiked chain but banning all armor of any kind, for example, can have balance issues). But then a rifle doing 5d10 damage shows up.
4) House rules that radically change how key mechanics work. In the old days, my favorite example was fumble rules closely followed by critical hits that did loads of damage. These days a system where arcane spells fatigue casters (see Unearthed Arcana) is my current favorite child for this sort of problem.
My main issue is that these changes, made for story reasons, are hard to do well. As a result, the game often ends up going very badly.
I like to tinker but I think the next time I DM I will make a point of not modifying any rules!
But it can radically change the balance of a game and make things a lot less fun.
In particular:
1) Mass banning of certain spells or classes of spells in order to enforce the campaign theme. This can go very badly, very fast. For example, no teleport spell seems like a reasonable choice. But then you decide it's a water themed world so only water or ice damage spells exist. And then you note that summons don't work including things like the mount spell.
After a while it can really effect playability.
2) Very restricted character classes -- a party can be awkward to balance if you discover no healing classes are allowed and so forth.
3) Restricted items and technology. Wizards can't get spellbooks (I watched this for 8 months once). Magic items are nearly non-existent. Standard weapons are not available to be purchased or used (okay, I can understand the spiked chain but banning all armor of any kind, for example, can have balance issues). But then a rifle doing 5d10 damage shows up.
4) House rules that radically change how key mechanics work. In the old days, my favorite example was fumble rules closely followed by critical hits that did loads of damage. These days a system where arcane spells fatigue casters (see Unearthed Arcana) is my current favorite child for this sort of problem.
My main issue is that these changes, made for story reasons, are hard to do well. As a result, the game often ends up going very badly.
I like to tinker but I think the next time I DM I will make a point of not modifying any rules!