Team Preview didn't entirely "kill" leads. I just took the lead metagame and made it one with the rest of the game. Dedicated leads still exist and they still do their jobs very well. With Team Preview, you now don't have to choose just one Pokemon to lead with when team building and your leads can "appear to be" in any slot of your team.
The timing of this thread is a little funny to me because over the past few weeks, after seeing the havoc that Deoxys-S brought, I've been testing all the old "leads" and seeing how they still hold up with Team Preview. I can easily run down a hand-full here:
Azelf: Although the Explosion nerf hurt, Azelf still does it's job of getting SR up and can still prevent the opponent's hazards from being setup with that fast Taunt. When Azelf's put on a team of hard-hitters, after that first Taunt, your opponent might not be able to get hazards up the entire game without having to give up a Pokemon. Very much the same way that Deoxys-S was doing earlier.
Aerodactyl: Pretty much everything that can be said of Azelf can be said of Aerodactyl. "Taunt turn 1, Rocks turn 2" still works as effectively as ever so long as Magic Bounce/Coat isn't around. Funny how Team Preview actually helps remedy that issue.
Infernape: While normally a dedicated sweeper in 5th Gen, Lead Infernape still does it's job surprisingly well. No common Stealth Rocker in OU enjoys being around Infernape's Dual STABs. Just like his 4th Gen counterpart, given a Focus Sash, he can deal with those common Scarf leads like Landorus & Terrakion if need be.
Machamp: This guy is STILL a great lead. Nothing enjoys a Dynamic Punch, nothing can seem to OHKO the bastard bar SE or Weather-boosted attacks. Using Machamp is basically free damage turn 1, but with no recovery he does get picked off eventually due to his slow speed so I can see why he's not so hot right now.
Metagross: The old Occa Berry Metagross still works pretty well. Hammer Arm is a little more needed than EQ with the addition of Balloon making Heatran even more annoying, but aside from that, Meta's still a very sturdy & powerful lead letting it get SR up as well as hitting hard right afterward.
Zoroark: While not in 4th Gen, I wanted to say this. With as many people in this thread alone that say that Team Preview killed Zoroark, while it's true that Zoroark would be a lot better without it, it's really not that bad for Zoroark especially if you use it intelligently. For instance, I run a Scarf Zoroark set which almost always is disguised as Celebi but I switch the order of my ENTIRE team around so my opponent would never know who it's appearing as since Celebi & Zoroark both appear in the middle of my team in preview. Celebi encourges Scarf Landorus to stay in & U-turn, letting Zoroark OHKO it with HP Ice. Zoroark U-turning against a Rotom-W reveals if it's Scarfed or not as if it stays in, I KNOW it's Scarfed & using Volt Switch so I can go out to my Ground-type & get Rocks up for free as Rotom's forced to switch-out. You get the idea. Zoroark gains instant momentum for me.
That's just a few but honestly, the rest of the leads, anti-leads, & momentum grabbers of 4th Gen are seen in the current metagame. T-tar, Heatran, Jirachi, I mean even their lead sets are common even though there's no Lead metagame. Like I said before; Team Preview didn't hurt leads as much as you'd think. You can still run a lead and even hide it in the 3rd or 4th slot of Team Preview, but unlike 4th Gen, it's not mandatory.
With all that said, I do enjoy Team Preview just because there's too much to even dream of being prepared for this Gen. You could have counters for every top OU Pokemon and a random Scolipede would run train over you. It's not THAT bad. I play PO's DW OU and doesn't use Team Preview yet it's totally playable and fun to be honest. I prefer Standard OU but the game would still be okay without Team Preview, you'd just have to scout REALLY well to be good at it.