Most people are banking on Liliana seeing a lot of playing once Ravnica drops. I guess we'll see. I'm hanging on to mine in the hope that she spikes in price. My LGS owner seems to think she will.
I mean, look at it this way. Wizards has been trying desperately to make mono black control a viable thing. So far, that hasn't happened. Rotation and Ravnica gives them a chance to do it, and I think Liliana is a huge tell that they're going to print some good shit for the archtype. I believe firmly that she'll be the best M13 planeswalker once Ravnica is out.
The problem with Ajani is he doesn't protect himself, and he has mixed abiltiies. Is he aggro, or control? He's super beatable in the current Standard and I don't really see that changing come Ravnica.
Normally I'd be inclined to go with most people, since some of them doubtlessly know what they're talking about whilst I don't, but in this case I'm quietly confidant that I'm right.
Lili protects herself even less than Ajani, comes down at a higher cost, with lower loyalty in a color that does a worse job blocking at the moment. She also isn't going to affect the board very much. Compare her second ability to ISD Lili. Both offer situational removal, but ISD Lili does it for one less mana and one loyalty less, and even she isn't that good.
Add to that the current unplayability of her ultimate. You're guaranteed to be at 6 mana minimum when it goes off, which means you should already be able to cast basically everything in your deck. Sure you could start playing things like Griselbrand or Diabolic Revelation, but the problem is that all those cards are completely dead up to the point Lili's ultimate fires, and most of the time it isn't going to fire.
She's also one of the only four mana Plainswalkers who can't win you the game on her own. Chandra, Jace, Garruk and Ajani are all win conditions in their own right. So is ISD Lili; something that cannot be said for her M13 counterpart.
Basically the only think she'll have going for her is the ability to go hunting for shock lands, I don't think that's good enough. The only deck I can come up with that makes use of her would be some derpy contraption looking to exploit Entreat the Angles and Bonfire of the Damned; and I'm quietly confidant that idea sucks ass.
If they're reprinting shock lands then I can't imagine that we're going to get a whole lot of other lands that could combo with her ability, so I'm not really seeing the potential.
Of course there's a reasonable possibility that I'm talking rubbish. Time will tell.