People always talk 60 cards this, and 60 cards that. Heck - even I say so. The truth? You don't always have to follow this rule, even if you think it is like the greatest sin a Magic deckbuilder can commit. However, sadly - it is true for online tabletop games optimised for Magic. Often, you find that the shuffler doesn't work so well when your deck is anywhere above 60 cards (even a single card is enough to find yourself having to mulligan like free), but when it comes to a 60 card deck, I'd say mulliganing becomes about a 10% affair (unless you really built a highly draw dependent deck, or are playing your worst matchup). In real life, people tend to argue a better number for decks is 61. (of course, we have the conservative group talking about 60, but that's irrelevant). I tend to agree - especially for non-combo decks. Strangely enough, 61 is a really good number. In real life, you do tend to randomize it better somehow. Many people attribute this to the fact that the deck can no longer be cut exactly in half - and such interleaving does reduce randomization within the deck.
However, that's not why I like it. I like it because it creates the most peculiar land-to-spells ratio. Just as switching out lands with spells and vice versa can change your mana situation complete, so can that additional card. In times when you find that your land is not enough - and putting more gives you too many - the simple act of adding another land to make your deck bigger might give your deck that magic land ratio. The one where it never gets mana-screwed, color-screwed, or any kind of funny land term you can think of. So yes, there are times when that extra land - WILL - save your ass. (of course, this applies for spells too!)
Then, we have the 62 card deck. I can't say I approve of this size, because it lacks the merit of uneven shuffling, but it is another number that works well with heavy mana acceleration. I use this number in my Mycosynth Golem deck - but no others so far. I do not believe that it is a very good, number - after all, what good can that extra 2 cards be? I won't dismiss it as a really good way to get to those ideal land ratios - but to stuff in extra cards - I'd say that's a real no-no.
After that, we have the greater numbers of 63 and 64. If you hit these numbers, chances are your deck's consistency will be hit badly. If it happens to be a quick aggresive deck, no problem - but control decks (besides those full of counterspells) and combo decks (and even Zoo decks - decks with a curve of creature at certain mana points, usually 1,2,5 and 6) will feel the pinch at this point from the lower consistency. When your deck needs to pull that Pyroclasm, Wrath of God, infinite mana combo, or even Enduring Ideal - you will know that the pain here is great. You will find yourself taking more damage, taking longer to put together your combo pieces and taking longer to draw that finisher card.
A great exception to the 60 card decks are decks built with a special purpose. Battle of Wits decks usually run 250 cards - and cry hard when Traumatized, especially if the deck isn't full of killer spells - which it often is. A deck running plenty of card drawing out of neccesity (such as for card advantage), especially in counterspell decks, might benefit greatly from a larger deck. The best advice, however, is simple: do not succumb to the temptation to put every card you thought might be good.
My next Magic post will probably about choosing which cards to cut! :P