What is a game feature?

Question

There is a passage in the DMG on p. 252 that talks about game features of the same name not stacking (emphasis mine):

Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them - the most potent one - apply while the durations of the effects overlap. (...) Game features include spells, class features, feats, racial traits, monster abilities, and magic items.

This list says game features "include" the items listed, but that does not mean this list is exhaustive. For example, it does not list equipment, nor does it list actions such as opportunity attacks, which are referred to in the Sage Advice Compendium as a game feature.

So, what would count as a game feature?

Answer 1

Anything and everything

"Game feature" is not defined as a game term. In plain English I don't think is ever possible that anything that causes a game effect is not a feature of the game.

It is also pretty evident that this is also the RAI of that passage at least to the point that I would scold any player at my table that tries to argue otherwise for wasting my time.

Answer 2

"game feature" is not a rules-defined term so it's going to be up to your table, your GM, your interpretation of English, and your philosophy on rules-interpretation

Honestly, there is little more to it than that. Lacking any given definition for a "game feature", other than, well, a feature of the game, there is little anybody can hope to go off of. Different tables will have completely different applications of this phrase and there is absolutely nothing wrong with that. Terms simply don't need, and often won't have, any sort of go-to, working, standard, ever-applicable definition.

In fact, the rules themselves are provably inconsistent in the application of this phrase. To quote the Sage Advice Compendium:

Q. Since game features of the same name don’t stack, does that mean a target can’t be affected by a shadow’s Strength Drain more than once between rests?

A. The intended function of Strength Drain is that it stacks with itself, as signaled by the fact that you die if your Strength is reduced to 0 by it.

You'll note that it says the intended function, which means it isn't the written function. It is abundantly clear that Strength Drain is meant to stack but we can see that Strength Drain is a feature of the game and thus actually wouldn't stack under various interpretations of the rules. It is going to be up to the table how they interpret the rules and how they either apply, modify, or ignore that interpretation.

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I personally can't tell you what I consider to be a game feature because I don't use a strict definition. All the tables I am in have gone on a case-by-case basis when something in the realm of corner-case-stacking came up. Most of the tables I am at, and have ever been at, have favored case-by-case rulings (with explanation and logic, of course), over hard rules. The reason we've done this is because hard rules often led to even worse complications and corner-cases. Almost all of my tables have determined and accepted that 5e's use of natural language actively lends itself best to a case-by-case, context-dependent analysis of the rules text.