At this nascent stage of this forum's life, I don't think we can afford to split off discussion into multiple subgroups yet. I think it's fine to gum up the main forum with individual card discussion for the time being. After all, who doesn't love preview season!
Anyways. I have no idea how playable the "world's best thief" is, as it's been notoriously tricky to evaluate new planeswalkers. But I will say that I love that ultimate.
Edit: On second thought, maybe he isn't all that difficult to evaluate, after all. His first two abilities are roughly analogous to Ajani Vengeant's, right down to the loyalty costs, and he even has the same converted mana cost. He has an extra point of starting loyalty over everyone's favourite cat planeswalker.
Having Lightning Bolt be his main defensive mechanism isn't a huge downgrade from Ajani's Lightning Helix. Indeed, that extra loyalty makes all the difference in the world when you're able to fire off two bolts in successive turns. The plus ability could be what makes or breaks Ral, and it's here that I think he falls a little short. Locking down their best creature or their most important land was a key component of Ajani's power, and Ral doesn't let you stifle their mana production in the same way. Instead, you can falter a key blocker, reuse your best land for mana, make your biggest creature vigilant, or mix and match all of the above. It seems like a fine ability, but not earth shattering. I'm probably the least capable brewer on this forum, though, so there could be a million uses for this ability that I'm missing.
Ajani Vengeant set the bar really high for planeswalkers, though, and even a poor man's version could be a reasonable inclusion.