This one is pretty big and I'm surprised it hasn't been discussed more.
First of all, imagine if the various labour and socialist movements called for a general strike to stop a major war, and failed, or even failed to call for one. There is no question that the working classes of Europe would be in much worse condition. You can forget about getting universal suffrage or the expansion of the welfare states, or the co-ops. The fact that the SPD and SFIO managed to pull this off demonstrated that the working class could come to power, and could do it peacefully.
And the Tsar would have remained in power a few more years, and maybe Russia would be an absolute monarchy to this day.
On the war itself, I've seen different opinions, and I think the really big variable is if Britain gets involved. They had an "understanding" with France and Russia and had apparently done military planning with France, but on the other hand there was no formal alliance, you had another Irish home rule crisis at the same time, and the British had tended to sit out continental wars if there was no Napoleon threatening hegemony. If Britain actually goes to war against Germany, they can cut off the German supply of nitrate, which came from a small island in the middle of the Pacific, and it would have been a pretty short war. But I can't think of how the British government would have sold entering the war to the British public, and the Liberal government at the time even depended on the Labour Party, whose support of the strike turned out to be the tipping point, for its majority in the House of Commons.