As I've been practicing my drafting, I've gradually learned some combos to look out for, and heroes to ban based on my opponent's picks. I'll add to this as time goes on.
Drow + Visage = very powerful familiars. Add Venge or SFiend for additional pain.
Drow + Windrunner = very high-damage Focus Fire (add Desolator on WR for tower melting)
Drow Precision Aura (or Lycan Howl) + Clinkz / Windrunner = very effective Strafe / Focus Fire
Spectre + Troll = very fast-hitting Haunt illusions (with max Desolate). Hell on supports. Requires dual lanes, or for Troll to go mid (preferably against a weak enemy mid). You might get away with "support Troll" if no aggro-tri against you.
Faceless Void + Witch Doctor / Jakiro / Lich = very good damage-dealing synergy with Chrono, plus defensive supports to protect a farming 1-position Void.
Note: it's far easier to ban out the Void than trying to ban out the multiple supports that are good with Chrono!
Wisp + Tiny/CK/Sven + Nature's Prophet = a very powerful global ganking combination. While you may not be able to prevent the Wisp + Carry, at least don't let them get both Wisp and NP! AA, Clockwerk, and Invoker are also good with this combo.
Lycan + Death Prophet = painful pushing power. Lycan runs around the back to demolish supports, while Exorcism goes to work on the rest of the team.
Lycan + Broodmother = Howl makes for very dangerous broodlings.
Mirana + Bane / Shadow Demon / Crystal Maiden = very good set-up for arrows.
Visage = heralds an aggro tri-lane (usually). May also prompt a Drow last-pick.
Vengeful Spirit = heralds a roaming support or roaming pair. Avoid leaving squishy carries alone if they don't have an escape skill!
Earth Spirit = heralds a WTF (just kidding!)
Disruptor + Axe / Undying = very mean "cage match" with Helix or Tombstone. Sometimes done as an offlane duo, or as part of aggro tri-lane. Jakiro also combos really well with this. Countered by heroes with escapes or healing supports.
Centaur / Shadow Shaman + Skywrath Mage = good control and good damage; very potent roaming pair
Clockwerk + Skywrath = put enemies in a box and nuke them, while interrupting them constantly.
Clockwerk + Invoker (Exhort) = Cogs makes the killbox, Sunstrike makes the kill.
Nature's Prophet + Skywrath = surround them with trees and nuke them from the sky.
Magnus + Anti-Mage/Phantom Assassin/Faceless Void = carry doesn't need Battlefury to farm effectively. Hard carry can do massive damage during Reverse Polarity.
Warlock + Undying/Elder Titan/Kunkka/Phoenix = it's bad when you're in a massive pool of syrup and can't get out of the way. Add Pugna with Nether Ward for additional enemy frustration. It's a good idea to pick another support with a teamfight ult so that the enemy has to choose between targets to interrupt. See also
Team Ideas around Delayed AOE.
Anti-mage or Phantom Assassin = may prompt a last-pick Magnus, for the "free Battlefury" from Empower.
Heroes that are Helped by Heroes:
Anti-mage -->
helped by --> Magnus (Empower = "free Battlefury")
Axe --> Disruptor, Undying
Bloodseeker --> Zeus (get targets low health faster)
Bounty Hunter --> Lycan (Howl), for massive Jinada strike
Clockwerk --> Invoker (Sunstrike), Skywrath (ult), Gyrocopter (Call Down), Lina/Lesh, Kunkka (Ghost Ship)
Doom --> Abaddon, Dark Seer, Dazzle
Dragon Knight --> Beastmaster aura, Crystal Maiden aura (DK needs AS and mana regen)
Faceless Void --> Dark Seer, Magnus (setup for Chrono)
Undying --> Broodmother, Nature's Prophet, Chen (numbers around Undying boosts Soul Rip)
See also: List of Initiators by Ability Type
