Cool. Why not flash so you could use it to Stifle?

Because I feared it would have a similar problem as giving Meddling Mage flash. A lot of players wouldn't know that to prevent Banishing Light from exiling something, they would have to respond to the casting of the spell, not the trigger. If this has flash, people might think that they could prevent Kozilek, the Butcher of Truth card draw trigger by flashing this in as a response, which doesn't work.
Then again, Wizards printed Hushwing Gryff, which has similar "problems."

Triggered abilities of emblems, of course, will still trigger.

Triggered abilities of spells? I don't think there are any of those, are there? Primal Boost etc count as cards when you use their cycle-triggered abilities, not spells.

­Hushwing Gryff is most naturally used in response to a creature spell, or a spell that would put creatures onto the battlefield. Which is a sensible way to time it. This... is harder to point to a typical use case, so Link's concern makes sense.

I was trying to cover all of my bases, Alex. Does Kozilek, Butcher of Truth's "when you cast" trigger come from a spell source or a card source?
I didn't want to call out emblems. Perhaps this should say "triggered abilities of sources your opponents control" so that it just encompasses everything.

I've said this before, but I wish comments were clearer whether they're critiquing templating, intent, or not sure. Assuming the intent is "any triggered ability", "triggered abilities from sources your opponents control" or "triggered abilities that your opponents control" seems a lot clearer if it works in the rules (does it?).

I think it does work, but I feel like it needs the word "sources." That might just be my own compulsion, though.

The Eldrazi titans trigger as spells, not creatures. I think the new template is fine.

I meant "spell or card," not "spell or creature." Oops.

Oh. Well, it's definitely a card. (Unless you're casting a copy of it somehow.)