I know Trundle's Ice Pillar counts as impassable terrain, but what skills does he block? I know about Shyvana's dragonform being blocked (happened to me yesterday) and I got intrigued.
Trundle's ice pillar counts as impassible ground as long as it lasts, so any movement abilities that would otherwise wind up inside the pillar will be shunted to either the far side or the near side (whichever is shorter). This means especially short movement abilities (such as Lucian's Relentless Pursuit, or Nidalee's Pounce) may not be able to "jump" the pillar at all.
(I would imagine that Shyvana's Dragonform should be able to cross the pillar, at least if she starts adjacent to it.)
Skill shots, including things like Blitzcrank's Rocket Grab, will simply pass through the pillar, just like casting a skill shot through any other impassible terrain (like jungle walls).
Situation: Red leesin is jumping at blue Anivia.
Trundle from either team could cast pillar to stop leesin as long as the animation for leesin has already begun, however if trundle casts pillar too early, leesin would skip right pass it.
Same case for Anivia, she can only stop leesin if she casts wall after leesin starts the animation for his dash.
This applys for all walls and dashes(Including Tristana's jump, Vayne's tumble and Quinn's vault).
Almost nothing, besides movement.
It doesn't block projectiles. They go through impassable terrain the same way they go through walls.
It doesn't block jumping abilities either. However, it will move the landing point if it was precisely where the pillar now is: in most cases, the jump will be shorter.
Impassable terrain created by an ability does not count as a wall for the purpose of abilities like Nautilus' hook (Q) or Poppy's charge (E).
It's the same as an Anivia wall, in a sense that it can block only movement
Trundle pillar is just like a condensed minion pillar. You can "jump" over it, like with a jax jump for instance. Your dragonform would be blocked, if your landing place would be on the pillar, and the game does not allow you to land in terrain.