1. Robb and Blackfish not being more specific about telling Edmure to hold Rivverun and not go and attack Tywin Lannister.
2. Renly crowing himself. Fucked everything up.
3. Ned telling Cersei his plans
4. Robert making Jon Arryn hand of the King. The hook up with Cersei alone proves he is the worse Hand ever.
5. Rhaegar taking Lyanna away, starting this mess in the first place.
Mance escaping from his cage and slaying Roose Bolton.
Stannis being reinforced by Tormund's wildlings and march on to Winterfell
Victarion arriving in Mereen to turn the tide.
Barristan slays Daario
Dany makes an unlikely alliance with the Ironborn.
Tyrion meets Dany for the first time.
Aegon makes himself known to all of Westeroes after taking Storm's End
Dany lands in Westeroes
The Twins burn.
Davos brings Rickon to Winterfell now in the hands of Stannis
Also I hope the Manderly men will jump the Freys on the way to march against Stannis and join him to take Winerfell. Ramsay would just head to the Wall
The advantages the Plains Indians had was they were fighting on their own land. If Robert Baratheon managed to hall ass across Essos to the Dothraki Sea then he would be screwed. The Dothraki never knew Westeros. The Reach seems likely to get burned the most, the Stormlands and Dorne would be all right. All they have to do is close the gates to Kings Landing. The Vale, forget about it, too many mountains, same with The Westerlands. The Riverlands can be easily defended, the Neck is swampy and a death trap thanks to disease, huge freaking lizards and Cranngomen. Then they'd have to get passed Moat Cailin. By then Winter will hit and they will freeze in their leather vests.
Oh not to mention the Ironborn. They can attack Dothraki hordes and get away by ship before they can hit back.
Unsullied have proven how effective they are against Dothraki attacks but they would lose 90%. It doesn't mean no army can face Dothraki without dire consequences, it must means Unsullied are only good at following orders. Defend the city. They did, even if it meant death. Westerosi commanders and the GC would have found better ways to defeat Dothraki.
All Stannis and Ned have to do is check out all of Robert's bastards and their mothers and see how they all had black hair even if their mothers were blond. To have 15 kids with black hair except three "trueborn" blondes. It was convincing enough for Ned, Stannis and Jon Arryn.
You can't make clear analogies with Dothraki and Nomads of the Steppe such as Mongols. The Mongols wore armor. In fact their armor was very effective. They wore armor of leather with silk shirts underneathe, arrows would have no effect on them because they'd get slowed by the impact of leather and twisted in a bundle in the shirts, acting like kevlar. The Mongols did not carry blades like the Dothraki which could be curved or straight. They could cut through leather armor easily. But the Dothraki arkah is absolutly useless against plate/mail armor. It is a slashing weapon. They can cut down pleasants, sellswords but a knight can slay dozens of them before he gets knocked on his back. They may be horse lords like the Steppe People such as Huns, Mongols, Turks, ect but they are very different.
Their bows are a different story. Drogo could be easily slain by Jaime or even Ser Barristan. The Westerosi knight is a killing maching and while they engage their foe they watch for weakneses, let them give a few blows to their sword or sheild, see what they are made of, then deliver the kill. Drogo would be dead no matter how many khals he has slain. Also he is no commander like Tywin, Ned or Stannis.
Tywin was hoping that Robb would meet him at the Green Fork. He thought that Robb would fall for his ploy because it was his first time in war. Instead he ended up facing Roose Bolton who did not fall for the trap and Robb destroyed Jaime's army because ironically he did what Tywin hoped Robb would have done at Green Fork.
The Blackfish was just bullshitting Jaime. The whole time he was planning his escape. If he recieved Jon's letter then he would know that Jon was not in league with the Lannisters by accepting Stannis at the Wall.
I think the Greatjon demostrated the opportunity the Northern Lords have been waiting since the last dragon died. The Starks kept the peace since then and since the rebellion Ned and Robert's friendship held the North and South together. The Lannisters thought getting rid of the Starks would keep their hold of the North now that Robb is gone. But it didn't. The Boltons only have partial control and they have been losing numbers to King Stannis. Without the fake Stark girl their position got weaker. They probably should have offered peace to Robb when they still had the chance to concentate on fighting Stannis. They were able to kill Robb but their greatest threat all along was Stannis.