Just a random thought of mine:
Stack Size is the total mana cost of a creature + enchantments. For example, an unbuffed Dark Pact Slayer is a 13-mana stack, while a Vampiric, Cheetah Speed, Bear Strength Lord of Fire is a 40-mana stack.
Everyone knows that Stack Size is the most important part of "delay" card effectiveness. Banishing an unbuffed Dark Pact is spending 14 mana to delay a 13-mana stack, and probably not a very smart move. On the other hand, banishing a fully buffed Lord of Fire is spending 14 mana to delay a 40-mana stack. That's pretty awesome.
However, you can also think of Stack Size as a modifier for damage and healing. For example, dealing 5 damage to an unbuffed Timber Wolf (10 hp, 9 mana) is "worth" 9*(5/10) = 4.5 mana. Dealing 5 damage to a fully buffed Lord of Fire (14 hp, 40 mana) is "worth" 40*(5/14) = 14.3 mana. On the converse, healing 5 damage from that Lord of Fire is "worth" 14.3 mana.
When you think of Stack Size as multiplying the value of all damage and healing, you begin to realize how risky it is to over-buff a card. The aforementioned Lord of Fire is so valuable that even highly inefficient attacks like Arc Lightning become cost-efficient. Your opponent has every incentive to nuke that mega-stack out of the sky, no matter what it takes. And when that mega-stack falls you lose 40 mana worth of cards.
For this reason, I always try to spread out enchantments between my creatures and my Mage. Stack size is a non-issue for the Mage, since your Mage is always a target regardless of what is on him - actually, having more enchantments on your mage makes it harder to focus him down.