The only problem with this is the fact that mastery is a skill you learn from your trainer at either 75 or 78 (I forget which).

You don't get your mastery until the seventies, I'm afraid. Sometime after you hit the bottom of your talent tree and are beginning to branch out into your the other talents.

Besides, you can argue all you want about how much a feature might help at lower levels, what really honestly truly matters is how it performs at level cap.

But why wouldn't they just make Shadowform the spell you automatically get when you talent into Shadow, then? Ferals get Mangle automatically at level 10, frost mages get their W. Ele, why not give baby spriests Shadowform?

They don't give priests Shadowform for their specialization because there just wouldn't be enough to do at that level. The 15% Shadow damage buff would feel like a drain, considering you'd have to pop out of form to Smite until MB came off cooldown. Which also brings into account all the extra mana of having to jump back in form... if it would even be worth it for the sake of one spell every 5-8 seconds.

Mind Flay at least gives Shadowpriests another shadow spell to use, plus a snare, to an extent.

I do remember that when shadow orbs was first mentioned it was talking about giving it to lower levels to make them "feel" like a shadow priest, prior to shadowform. I think it would still work nicely for low level spriests, they just wouldn't get the extra buff from Mastery until much later in the game, of course.