This is not about the design or mechanics of a game, but in the way
the actual physical bits are produced. There are ways to do it
politely, or not.
For example: you produce the core set of a game. Later, you
produce an expansion, with the intent that the cards be shuffled with
the original game's cards. It is possible that at some point someone
will want to take their shuffled cards and separate them again:
perhaps they want to teach a novice the game, or perhaps they want to
sell game or expansion separately. So it is really helpful to put an
icon on the card which lets the owner visually sort out the expansion
cards from the originals, without having to check a list of individual
card names. It doesn't have to be big and obvious, because it won't be
used in play, but it should be near a corner so that it's easy to spot
when sorting cards.
Similarly if you are issuing errata (I'm looking at Ashes: Rise of
the Phoenixborn here), there should be some way of looking at a pair
of cards with the same title and determining that this one is a more
recent version than that one. What with the Reborn set and later
errata, there are cards with three and maybe four versions out there.
How does someone who buys a second-hand set work out what they've got
and what they might need to buy to upgrade? So please, at least on the
replacement cards, put a version number or date or something. Again
this need not be huge, but it's very helpful if it's there.
And while many companies are doing better at this these days I still
need to plead for the colour of components not to be the only way of
telling them apart. Yes, there are ways of choosing colours to
maximise the number of people who can distinguish them, but if at all
possible please put a distinctive symbol, or shape, or something
else too. (A good example: Sea Salt and Paper has a colour icon on
each card, so that you can immediately tell whether two cards are "the
same colour" for game purposes. They use the weird proprietary
ColorADD system who won't even tell you how much it costs for a
licence, but you can't have everything…)