@Deuce: some valid points concerning the disappearance of the Steam Tank in 6th edition. I don't know if the later army books had such a big cut in their army list (and characters) as well, or that it truly were the earlier ones.
It varies quite widely and it's not always easy to tell, as 6th edition did some general reshuffling in most armies which saw some units renamed or otherwise changed about a little and some armies were carved up completely. Pretty much every army lost some units if nothing else because of the removal of generic monsters and swarms between editions.
On an army-by-army basis:
Bretonnia: Lost no units (though Squires were renamed). Gained Pegasus Knights, Grail Reliquae and Trebuchet.
Chaos Dwarfs: Never released by the Studio.
Dark Elves: Lost sword-armed Warrriors and the hybrid City Guard unit (spearmen and crossbowmen) though the spearmen and crossbowmen themselves were retained as single-weapon units.
Dogs of War: Never released as a standalone book. WD list published. Lost no units, gained an army's worth of non-RoR units.
Dwarfs: Lost no units. Gained the Engineer and Rangers.
Empire: Lost Outriders, Foot Reiksguard, Kislev Winged Lancers, Kislev Horse Archers, Dwarfs, Halflings, Ogres, Steam Tank, War Wagon and the Halfling Hot Pot. Three Knightly Orders merged into a single unit list entry with upgrade option. Gained Free Company, Templar Grand Master, Engineer, Warrior Priests.
High Elves: Lost sword-armed Warriors (may already have been removed in the 5th ed rerelease; I don't have that book).
Lizardmen: Lost Skink cavalry and some weapon options (bow-armed skinks). Gained chameleon skinks, Saurus Cavalry and the Carnosaur.
Orcs and Goblins: Lost Forest Goblins, Forest Goblin Spider Riders, Ogres, Snotling Swarms and orc-crewed static war machines. Big'Uns, Squig Hoppers and Netters become upgrade options rather than discrete list entries.
Skaven: Lost the Vermin Lord and Doomwheel. Gained the Master Moulder, Warlock Engineer, Ratling Gun, Warp-Lightning Cannon and Clan Eshin Night Runners. Assassins go from regiments to characters. Warpfire Throwers become a unit upgrade.
Undead were discontinued in their generic form but all units divided between Vampire Counts and Tomb Kings armies; none lost overall. A handful of units gained, mostly on the Tomb Kings side.
Vampire Counts: No units lost or gained.
Wood Elves: Lost Chariots and Glade Guard (name reused for former "Archers"). Scouts become an upgrade option rather than discrete unit. Gained the Eternal Guard, Great Stag, Branchwraith, Treeman Ancient and various character options (Waywatcher, Wardancer etc.), Wild Riders and Treekin.
Chaos is obviously tricky as those lists were broken up completely. Overall the armies gained a number of units and lost few if any (Centaurs, arguably, but Centigors were a like-for-like replacement)
From a casual glance it does seem that the first two armies out of the gate (Empire and O&G) were hardest-hit by unit losses and are probably the only armies to have "net" lost units, although they did get a couple of new toys (O&G effectively gained two new units out of losing Big'Uns, by 4thed standards; Empire detachment rules). It's also hard to assess the loss of a regiment vs the addition of a new character Later armies, like Wood Elves, seem to have done much better out of the deal but that's partly because those armies were extensively redesigned anyway and the long wait arguably hurt them more than the new units helped.
It's worth noting too that this only covers the first release from 6th edition. Later army books and upgrades would introduce new units, reinstate some of those lost (like the Steam Tank, and some of the O&G units), but I can't be bothered to work that out right now.