So why does it make sense that Varian should be High King? Besides being a human, of course.
Lor'themar is not just a throwaway character that gets a shot at Warchief just for showing up. He's a potential candidate because there are several very compelling reasons why he could be, and
no reason why he couldn't be. Is this a 100% sure thing? No of course not, it's almost impossible to predict Blizzard's final decision on these sorts of things, but it's unfair to say he can't be warchief just because he's not an orc, or he's an elf, or he's not the warchief for the Horde.
Some common points I'm seeing:
"But Lor'themar wanted to join the Alliance!"
Because when your people are being persecuted in a way you've sworn to never let happen again, the best decision is to just suck it up and take it, right? Lor'themar wasn't leaving "our" Horde, he was trying to leave Garrosh's True Horde. Lor'themar was simply out of options. In fact, this little argument actually works in favour of Lor'themar for Warchief.
It has to be understood that becoming Warchief so far has pretty much been a "yup, you're it now" affair. For the first time since Doomhammer took the title from Blackhand, someone other than the previous warchief gets to have a say in who it will be. The new Warchief HAS to be someone both factions approve of, I mean, that's the whole point of both factions taking part in this. Now who on the Horde has proven himself in combat and leadership against Garrosh and has also procured powerful blood golems for the Siege when he led the assault on one of the most powerful tyrants Azeroth has ever seen... Huh that would be Lor'themar, I guess he has done a lot for the Horde recently... and all of Azeroth really. I SUPPOSE the other Horde leaders respect him, why wouldn't they? Oh yea he's a blood elf, so of course they just think he's an effeminate boy toy. OK so who on the Horde would the Alliance approve of? I mean, they hated Thrall and he was as diplomatic as you could get! Oh! How about the guy who was just in talks with you about rejoining his people into your faction? I guess that's also Lor'themar... huh.
"But Lor'themar only cares about his own people, he couldn't care less for the other Horde races!"
There is nothing supporting that he doesn't care about the other races. He has done what he could for his people because he CAN. What he could do for the trolls and tauren in Orgrimmar is pretty much out of his hands. Blood Elves were persecuted just as harshly as trolls and tauren, maybe even more so, where were Vol'jin and Baine then? Oh, I'm sure they would help if they could, but it really looks like they had their hands full at the time looking after their own people... similar to Lor'themar.... HUH.
"But he's a blood elf, blood elves are too feminine and not savage enough to have one as the leader of the Horde!"
Uh, he would JUST become Warchief, this doesn't mean that suddenly all of your favourite orcs, trolls, and tauren will suddenly be more feminine and less savage. People also like to just think about orcs, trolls, and tauren when they say this, and completely forget that the Forsaken, blood elves, goblins, and pandaren are also part of the Horde, and are not lesser to the first three. The Horde are still THE HORDE, you just have someone different at the top. Remember when Garrosh became warchief and all of the players totally took his word for what the Horde really was? Oh wait...