There was an interview with Halo Wars developers, who said that they made Mark IV more intricate and majestic on purpose. Mark IV was the armor made early in the war, when the UNSC could afford a little flash. The later Marks are streamlined and utilitarian in comparison. The '57 Chevy seemed like an apt comparison, though I must admit that I love the style of the truck from the 70's and 80's more.

: There are a few distinct variants, particularly those that have remained
: pretty faithful to their original incarnations, that still look great
: despite little design tweaks. Air Assault, ODST, Recon, EOD, and Scout,
: among others, are still worthy of sitting on my virtual super duper
: soldier head.

Nope. Air Assault was my absolute most favorite helmet in Reach. The Halo 4 version is like seeing the later production run of a beloved car, shortly after the workers have been informed that the factory is closing down and the CEO has absconded to the Maldives with their pensions. The body work is ugly, the panel gaps are horrible, and I can't figure out why someone would do that to a perfectly good design.

: Most other variants, particularly those introduced in Halo 4
: or legacy designs completely revamped, don't look like they have any place
: in a military setting.

Yes. This.