There is no cow level.
Repeat that three times.
But if I was designing WoW I'd make sure there'd be some kind of cow level. After all who doesn't want to slaughter some bovines from hell?
"What's a good reputation to grind for a newly 80 Ret Paladin?"
First get up to friendly with the Ebon Blade so you can get the Tabard of the Ebon Blade and start championing them in normal and heroic dungeons. Ebon Blade rep will give you the head enchant Arcanum of Torment. There are also a few dailies (and a lot of rep) out in the Shadow Vault, but you have to follow the Ice Crown quest chain until you get there. Only take a couple hours of playing though to open them up.
While you're championing the Ebon Blade, you can also be grinding Sons of Hodir rep for your shoulder enchant. If you're not an inscriptionist (or scribe if you prefer to want to complain on the internet about it) then you want to be sure to get your Hodir rep up as fast as possible – it'll be key in any gear check to get into raids.
"Is there a cow level?"
There is no cow level.
But wait! There's more.
In Diablo II there was a secret cow level that you could access once you completed the game. Back then level meant more of a Super Mario level than a "level 80" level. Today people think that a cow level is something like level 80 (or 70, or 60). But there is no cow level in WoW. The history of it is interesting though.
However... in DII the Cow King drops a chest piece called the Cow King's Hide, which also happens to appear in WoW.
"Any ETA on the faction change mechanic?"
No updates since the original announcement, however I wouldn't be surprised to hear one soon. I'm sure we'll hear more about it at BlizzCon, one way or another.
"What would happen if I logged a character out in the middle of the Argent Coliseum while it's being constructed and then logged back in after it was finished when 3.2 went live?"
You'd probably just be inside the walls and could walk right out. But really I wouldn't do this. If you get caught intentionally bugging or subverting game mechanics you'll get slapped with a warning at least, or a ban at the most.
And if they don't do anything punishment-wise to you, and your character is stuck, well... GM queues are ridiculously long right now, so you'll probably be stuck for a while.
"What's going to happen to my glyph of Seal of Blood on my pally since they are getting rid of the ability? Will they just remove it from my glyph book, or will it turn into something else?"
Mine just disappeared and showed up as a blank space when I moved my toon to the PTR.
"Is there a way to speed up resting time?"
No, not beyond the normal resting bonus you get at an inn.