Interesting ability, but I worry that it will lead to too many feel-bad moments for its controller, e.g. they exile all their lands and their opponent Doom Blades this. The lands won't come back unless this is on the field. An alternative that gets around this: combine the two abilities to create a delayed trigger, e.g. "Exile a tapped land you control: Target creature gets +1/+1 UEOT. Return the exiled card to the battlefield under your/its owner's control at EOT." Or you can make it a triggered ability, e.g. "Whenever you tap a land (for mana?), you may have target creature get +1/+1 UEOT."

I knew something was wrong, but I couldn't see what it was. The delayed trigger is a good idea.

Part of the problem is that I'm not sure what this card is supposed to be doing. Is the point of the card a sneaky way to untap lands?

If it is, then there's a problem with doubling the amount of mana in your pool during any player's end step. Tap and exile a land during your post-combat main step. That will set this guy to "At the beginning of each end step". When the trigger comes around, tap all your lands, adding the mana to the pool, then exile them. They'll all pop back into play, so feel free to cast your Searing Wind with 5 land on the board. In a 4 player game, you should be able to do this twice before your turn pops around...

jmg: Nope, mana disappears at the end of each phase. This isn't going to enable tricks like Sword of Feast and Famine.

However it does seem pretty scary. It's like an Ursapine that accepts any colour of mana and doesn't stop you spending the mana on something else.

jmg is right in that it also acts as a land-only Seedborn Muse, though that can be worked around by having the lands enter tapped. You don't get the lands back if you activate during the end step until the following end step, so that degenerate interaction doesn't exist.

­Ursapine++ is an issue. What if something limited how often you could activate the ability? Maybe bring the lands tapped during the upkeep, so they don't untap the turn you use them?

Hmm... the point was actually to untap the lands. I've just increased the cost for now, but I'll ponder the alternatives.