Matt Tabak said, "creatures with power 4 or greater have flying." wouldn't work as expected. Can you explain why?
It’s not that it wouldn’t work, it just would work really really unintuitively.
Let’s try a little thought experiement: You control the enchantment with “Creatures you control with power 4 or greater have flying.” You also control these creatures:
Vile Aggregate (and three other colorless creatures)
Crater Elemental (with its formidable ability resolved)
Timberpack Wolf (and two other Timberpack Wolves)
Hooded Hyrda (with five +1/+1 counters on it)
Calcite Snapper (with P/T switched through its landfall ability)
Siege Rhino (just Siege Rhino)
Which of these six creatures would you expect to gain flying from the aforementioned enchantment?
Well you’re wrong. Out of these six creatures, only Siege Rhino would have flying.
Welcome to one of the major quirks of the layer system. Because of how it’s set up, effects of different types are handled in a specific order.
(That order is copy effects, control-changing effects, text-changing effects, type-changing effects, color-changing effects, ability-modifying effects, power and toughness -modifying effects.)
This means that the effects that modify things in earlier layers can’t look at things from later layers without giving unintuitive results.
When our hypothetical enchantment goes to grant flying to creatures, we haven’t applied any power and toughness modifying effects at all, so our example creatures don’t have any of their buffs applied and it sees them with the following stats:
Vile Aggregate as 0/4 (Its power is undefined, so 0 is used instead.)
Siege Rhino as 4/5 (smug bastard…)
So yeah, that’s why it wouldn’t work. It would only work for creatures with an actual printed power of 4 or greater and ignore any other effects that would modify that.
(BTW, the first five creatures are each examples of the five P/T sublayers. These are all the classes of abilities that wouldn’t be taken into consideration. In order: characteristic-definining abilities, power and/or toughness setting effects, increases or decreases to power and/or toughness not from counters, increases or decreases to power and/or toughness from counters, and power and toughness switching.)
