"Activate Jace +2."
"It gained loyalty, so this triggers on top of the fateseal. Add another counter."
"It gained loyalty again, so this triggers again. Add another counter."
"..."
{+2}: Draw the game. Such a bad planeswalker.

... Clearly not what is meant to happen. Rewording. Also ETB realllly shouldn't count as gaining, but I know it does.

I think what you want is for this to not be a triggered ability ("When" / "Whenever", which leads to the problems SM points out), but a replacement ability ("If a planeswalker would gain loyalty counters", which can only apply to any given event once).

That's... a really subtle and stupid wording problem.

Subtle, perhaps. But it's pretty necessary for the rules. Some things really need to use the stack and happen after their trigger events, and others really need to modify the event before it happens.

Without the first category things like Nekrataal and Aether Adept can't use targets and the stack, unless you do very odd things like make the creature spell targeted; without the second category things like damage prevention would have to do horrific retroactive things.

While we're talking about subtle, silly things... it seems odd to me that this doesn't just say "When you activate an ability on a Planeswalker, put a loyalty counter on it." Granted, that would give the Planeswalker a loyalty counter when you activate the -1, say, but I don't see a real problem with that. Just gives the card more relevance. Either way, Alex's comment stems from the fact that this card is altering the number, instead of just adding a loyalty counter, which seems easier to process anyhow.

Mmm, that implementation of the idea works, but is a bit different to the top-down I was aiming at.

Both the creative concept of "loyalty" and the mechanical concept of "more counters, less 'sacrifice'" seem an odd fit for black.

What you were looking for was Carth the Lion