Well, the flock of posts that are going to succeed mine are all going to directly contradict what I'm about to say. But let's address it anyway.
The fact is, most people's decks, I'd say more then 99% them, are dependant on a certain combo(s). There's only going to be a few cards to grant you your win condition, and the more cards you have in your deck, the more turns it's going to take you to reach them. So limiting you deck to the bare minimum will get you to that win condition that much faster. It's the reason you'll see people run cards like Gather Your Mind or Toon Table of Contents in a non-Toon Deck. The cards search out each other and thin the deck out making your combo that much easier to get to.
With that said, here comes the part that people have a hard time dealing with. Thinning your deck out, keeping your cards to the bare minimum is, like everything else, a strategy. Not a rule. I've beat 40 card decks with my 60 card deck plenty of times. And the reason why, is strategy.
Not everyone builds or plays the same way. When you build a deck, you shouldn't just throw in cards together because they look like the have a good effect. What you want is a combination of cards that have solid structure; cards that have synergy with each other. For most people, that means keeping the deck thin and manageable. But for some people, like me, that means that my structure may depend more on card interaction, then on card count.
Now, the following posters will, no doubt, just reiterate what I said in my initial paragraph, and contradict what I've said in my later ones. But it won't change the personal experience I've had with card count. From personal experience, I've found that higher card counts, with a view towards structure, worked better for me then the strict thin deck. Some of my decks are 40, simply because that's the interaction I want. But some are 50 or 60 because the interaction is something far more complex then the average deck. It's all in what you want to do.
Just remember, throwing a bunch of cards in a deck because you like the effect will do poorly in most situations. Most newer players do this with no regard for whether they have a strategy in mind or not. That's why a lot of vets don't understand the concept of the higher card count strategy. They started out with a giant size deck, and of course, didn't do well because of their inexperience. This has turned them off the concept of high card count altogether. But what's important is not so much the card count, as it is the strategy, structure and synergy of what your trying to do. If it works, and works well, then card count is not going to matter.
