I knew that immediately would be common, which is why I pointed it out. Unlike the last two editions, 5E was built with the assumption of modularity and house-rules (from developer comments during the playtest). 4E was perfectly balanced, which was both it's strength and weakness, but it made house-rules very dangerous to implement, since it would disrupt the very carefully designed balance. 3E wasn't really balanced, but this was the very height of the era of RAW, which I suspect was a byproduct of 3E. AD&D 1E was created to serve as a uniform set of rules for OD&D, which was rife with homebrew and house-rules. I'm not that familiar with BECMI (I mostly just used the adventures, which were mostly excellent), so I don't know how well it was customizable. The closest system to 5E is AD&D 2E, which was also readily customizable, and many ideas were eventually made official in later books.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.
@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)
As for how 5E is customizable, it's built on a pretty simple frame. There are official variants in the PHB and DMG, but there are endless options that can be easily added (or in some cases, removed). The DMGuild is an example of this.
Adding new monsters shouldn't count as homebrew, since there is a section in the DMG discussing how to do it. Assuming the rules are followed, this should still fit into RAW.@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.