(Idle curiosity... Are you the same Weirdo from the ADB/FedCom Forum?)
And Starship Combat News,
and CGL's Battletech forum, if you go to any of those. 
Oh, you want more detail?
Yep.
As things stand, I still don't know exactly what's in Unity that's different from AE. And my budget's tight enough these days that I'm not getting anything sought unseen.
...I suppose the rules modifications COULD have been presented in Rules Annex II, but for a couple things:
1) That still would have required you to buy another book.
2) Presenting it as "(Admiralty) Rules Annex II" would have implicitly maintained the Admiralty/Nova split, which was the primary problem this edition is meant to address in the first place. It is called UNITY, after all.
From my perspective, you've yet to list a downside. My biggest priority here is to NOT move away from AE.
What can't you live without?
A) A finely-crafted rules set taking what was best from each prior iteration of the game and building something even better. A comprehensive sequence of play is provided, and the rules are fully cross-referenced.
B) Our most streamlined and balanced construction/point-costing system to date.
C) Dedicated seeking weapons.
AE has all of these. Okay, Seekers aren't technically weapons, but I like that right now they don't eat up one of my extremely limited weapon types.
D) Six brand-new scenario templates, adding to the six from previous editions.
E) 48 sample ship displays from 14 different fleets across five separate universes.
If that isn't worth $13, I don't know what else to tell you.
Okay, the scenario templates are cool.
Which universes? If they're ones I already play in, you've got my attention. No idea where you'd get the licenses, though.
Honestly, the only thing I thought AE was missing was a way for ships to support each other, kinda like the ELINT and shared point defense rules from B5Wars. As is, there really isn't anything an escort ship can do to protect another ship aside from shooting at his buddy's attackers, and hope that convinces them to go away.