Name one good thing of each expansion

[Otiswhitaker]

You're missing the point. Those things you mentioned (transmog, etc) were included during a specific expansion, so they are counted as part of that expansion. The fact that the functionality extends beyond that expansion doesn't somehow invalidate that fact.

The fact that at no point it required you to pay for the expansion should :x

[6626]

BC: Heroics, flying
WotLK: LFD, WG
Cata: LFR, transmog
MoP: Too early to say

[Gigapants]

Vanilla: Naxx
BC: Sunwell and SSC
Wotlk: Phasing, which was the stepping stone for more theatrical questing experiences
Cata: 1-60 re-vamp
MoP: Account-wide stuff