I ran foot lists in 5th with the following:
and won more than i lost. Most of these units treaded water or improved. Haven't resolved the psyker powers/ avatar conundrum yet. The divination and telepathy power set could be pretty good for eldar, but fortune and doom are so good...
The biggest problem for a pure foot list is access to scatter lasers. With the nerf to guardians, there's no good place to get a scatter laser outside of the Vyper, WW, and Wraithlord... but all of those have drawbacks - namely, even three wraithlords only gets you 3 scatter lasers, which isn't enough. War walkers are fragile as all hell, and vypers are just as bad (although, OT matters less, as they will get glanced to death by bolters).
I might be able to make warp spiders work for S6 spam. I think the worst mission will be FA, because, even with buffs to Warp Spiders, our FA slot sucks, even more than our troops slot does.
I don't think swooping hawks will be viable. Even with the buffs to haywires, they're expensive and too fragile to be in assault like that.
Rangers and a doomseer are death on special weapons guys not in vehicles - sergeants and ICs in units can use the Look out sir! rule, but special and heavy weapon guys cannot.
Reapers got way better, both in a light AV-threatening role, and in anti MEQ, as hullpoints lets them kill transports on glances, and 5+ cover lets them hit MEQs... but some things still grant 4+ cover, so it remains to be seen just how often units will be in 5+. Focus fire is also a bonus, as it lets you demolish the half of squads not in cover.