So I was playing Greyjoy Mill against Stark/Tully, I had two games and in both games I managed to mill my opponents deck Entirely by turn 6/7 but both times I lost by the thinnest margin, once on a single dominance and once on a single Distinct Mastery.
After the game I found out that my opponent was running a 70 card deck! I was Not amused, were his deck 60 card I would have milled him two or three turns earlier and probably had no problem winning!
I thought that this was just a downside to mill in casual play that Id have to get used to but then I read the tournament rules and it turns out that there is no max deck limit in tournament play either! I realise that Mill isnt really meant to eat your opponents deck (negative player experience and all that) but its still one of the stronger aspects of it and you cant ignore it, am I the only one who feels a bit screwed running a mill deck when all your opponent has to do is have an extra few cards and all of a sudden you have less of an impact on the game?
First of all, the "Spoiler Space" forum is really about stuff from the books/series, rather than about gameplay, but I'll still respond to this.
You shouldn't feel "screwed."
I'm going to assume, since you used the term "mill" to refer to a discard deck, that you have a background in Magic: the Gathering, where decking out results in you losing the game.
I'm further going to assume that this is a tactic you like to use.
Maybe because of that you assumed that draining someone's deck in GoT would be an almost assured win. I'm sorry that's not the case, but it's no you getting screwed.
There is only one way to win AGoT: collect power; it is the only win condition (now that Doomed cards are only legal in Legacy). If your deck cannot do that, you cannot win.
What's the point of draining your opponent's deck, then? Simply, denying them options. If you have no deck, you're obviously not going to play any new characters, attachments, events, or location, except what you've got in your hand. It's a tool to making your position stronger, not a weapon to win the game with. And there's nothing wrong with that.
Why is there no max deck size? Because it is it's own drawback. Having a bigger deck means you are less likely to draw any particular card. Trust me, I used to have a Stark deck that was close to 200 cards. The only thing that made it work were all of the fetch mechanics in my plot deck.
The point is, I don't think you should feel "screwed" because a certain tactic doesn't work that way you want it to. It's a game, it has rules, and those will likely be different from other similar games.