Thanks to everyone who has responded and/or voted so far.
Some remarks:
I didn't think "immediately" would be such a common response. I would've included it as an option on the poll if I suspected it would be.
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Shiroiken : You wrote that 5e is great for customization. Out of curiosity, how would you compare the customizability of 5e to earlier editions, assuming you have experience with any of them? (open question if anyone else wants to respond)
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Satyrn: You mentioned inventing new monsters doesn't count as modifying. I respectfully disagree, but it wouldn't surprise me to learn I'm in the minority in defining homebrew content as such. The distinction for me is whether the
thing implies consideration of mechanical impact on game play. Something to do with the mystical developer's "stamp of approval" some GMs prefer or require before giving a
thing serious consideration for inclusion in their own games. Not that anyone needs WotC's approval to modify the game and have fun doing it, and anyway, custom monsters are some of the lowest-impact form of house rules (again, as I define it). Even so, I'd like to avoid derailing the thread with a debate about semantics. However you and others approach the question and select an answer is fine by me. Clarifications in written responses are appreciated.
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Jer: I hope my explanation to Satyrn explains the difference between the thread title and poll question - that is, there's no difference as far as I'm concerned. I did state that rulings on nebulous systems ("situations... that aren't explicit in the rules" in your words) should be excluded. If you feel otherwise, that's fine. I'm not going to try to police the thread, so again, people can answer the question/poll as they see fit. Also again, clarifications like yours are appreciated.
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ad_hoc: You wrote that it's impossible
not to house rule. Adventurer's League players, in theory, should all be operating under the exact same set of rules. A person who has only ever DMd AL would have a "Never" response if they're abiding by AL guidelines. That's not always the case, of course. However, as defined in the OP, rulings are not house rules (see response to Jer).
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redrick: You identified an interesting grey area - codification of a ruling. I feel there's a difference between codification of a ruling and mere consistency with a ruling. DM wiggle room, I guess? Not sure where I would place codification if house ruling is a binary yes/no situation. I'll give it some thought, and perhaps other participants in this discussion can weigh in on the point in the meantime.
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As to my own experiences...
Started playing D&D back in the 90's. Didn't get much gaming in '99-'14, then returned to D&D as a DM in Jan '14 with 4e. I gave it about two weeks before I started house ruling and it got out of control fast. When 5e came out that September, I immediately switched systems and did my best to curb the instinct to take a hacksaw to the game. After familiarizing myself with the system through some 1 on 1 play with a buddy, I stepped up as an AL DM for my FLGS. For the next 8 months, I ran games stringently by the RAW, and frequently sought guidance on the WotC forums when they were still a thing.
Once I left the program, I felt I had acquired a body of experience sufficient to better understand the impact any particular house rule would have on actual play, and let my inclinations resurface. Since then, I've run several campaigns with varying degrees of house rules, one of them unmodified aside from homebrew monsters, as Satyrn described. Of these, I found this one the most enjoyable to run for various reasons, and it has been my experience that, while 5e is a robust system capable of withstanding the stress of reasonable house rules without breakage (whatever reasonable and breakage means), it's most fun to run with little or no modification.
I actually responded "less than 1 month" on the poll because, technically, my buddy and I did a few house rules as we adjusted our mentalities to the new system. However, those were quickly abandoned once I got into AL, and I never returned to the modifications we invented.