Inspired by Innistrad, zombie movies and halloween, this is another example of the deck limit number - in this case ∞ (any number) for Challenge # 019

So each card has its own limit, instead of the standard 4-of rule?

correct, there would be a little deck/card symbol with the number and each card would have its own limit.
It has pros and cons -
Pro: More design flexibility (a slightly weaker common could be offset by having more copies available)
Con: Slightly more complexity during deck construction/tournament registration.

I am sure there are more pros and cons but they were the first ones I thought of (and they are only examples)

One other con: It would make Elder-dragon formats less viable.

Still, I can see this working. We've already got cards with 1 and &inf; here.

It would be interesting with EDH, some cards would get weaker, others stronger - remember, you don't need to have the cards reference their own DL numbers like my two examples (they were just a splashy way to work it).

But if more cards change the default, then changing the default itself becomes a lot less interesting as a format.

Arguable. EDH has a lot of strangeness going on... the Highlander aspect of it is just one of them. I assume, if this method was put into place, that EDH would feel even weirder, since you'd be giving a property normally reserved for Mythic Rares (You can only play one) to cards like Naturalize (which, I'd assume, would normally have a limit of 6, say). I am probably getting too picky, though, so forget my blather.

There are plenty of pros to this, which also include: Making Mythics feel more Mythic, allowing more cards that are fine if there is only one on the table, and grating if there are more than one (hello Night of Soul's Betrayal and Krark's Thumb). Not to mention lines of text like "If you have seven or more copies of ~ on the battlefield, you win the game."

What I'd really like, and I know that this is going well outside the range of "Reasonable things that Wizards would do", would be a string of numbers for each format. One large number for Standard, another diminishing number for Modern, then vintage, then legacy. It would give a feeling of "The stuff that is currently coming out is more relevant", while still imbuing a little "but you can still play with one in your deck, even if it is 7 years later, just because you love the card that much".

I like the idea as envisaged by Camruth.

I think jmg's last suggestion is a recipe for instant hatred by vast armies of kitchen table players, followed quickly by "Nobody pays any attention to those numbers except the first, right?" One of the things about Vintage / Legacy and casual formats based on them is that you can still play your old Standard deck there.

Le sigh. It's true. There are a lot of Magic players with a lot of different opinions. Trying to tell them to play the way I would like to play is much too trying.