Counts As is a rule used to justify both epic models and Games Workshop's tendency to cancel/not support armies with existing fanbases such as Lost and the Damned. The fix is easy: the old units now "count as" some other unit entirely. Sometimes it works; other times it does not.
Still other times it makes no fucking sense whatsoever, such as when GW cancelled the Codex: Eye of Terror after it had been used in tournaments, leaving a bunch of players with mutant-loaded Chaos Renegade armies with nothing but the fucking "Counts As" rule to keep them company.
The Other Side of the Coin
In a different light, for 40k, the "counts as" rule is a converter's and fluff-heavy player's best friend, as it can add a bit of individuality to an army. For example, modeling a single heavy weapons guy to "count as" a heavy weapons team in an Imperial Guard command squad, assistant Techpriests instead of servitors for a Techpriest or Techmarine, an Eldar sword painted glowing red to count as a Fusion Gun as well as a Power Sword, ect. are little ways this rule adds character to a model. On a larger scale, some Iron Warriors Chaos Space Marine players are known to use the loyalist Space Marine codex to better represent the mechanized organization of their legion than their current Chaos Space Marine codex can.
It also allows players to field fluff-based armies in more creative ways - say, for example, an Imperial world that has been cut off from the rest of the Imperium for thousands of years has slowly forgotten the superstition surrounding machines and has evolved to the point of re-manufacturing grav tanks and the like. Such an army would be Imperial in flavour, but would play using the Eldar codex. And in fluff, would be targeted by
Inquisitors the Mechanicum on a daily basis. Another common setup is to kitbash Imperial Guard and Tau models together to make a Gue'vesa force, counting the crunch as Imperial Guard or Tau, depending on what best represents the force's combat style and composition. Players with these kinds of armies are rare, however, so "counts as" mostly adheres to the first two types of examples.
As with the no longer supported models above who "suffer" from the "Counts As" rule, (and for anything in general) it's always necessary to inform your opponent as to what each of these changes count as.
If you followed this link expecting "Mandrakes" you might be a little confused. You see, 5th Edition Mandrakes used to be pretty much the epitome of this concept. They were one of the worst units in Phil Kelly's 5th edition Dark Eldar Codex but the models are pretty awesome, so most fa/tg/uys just used the models for something else (claiming Counts As on the result).
Anyway, in the 7th edition codex, Mandrakes become far better. You may now stop using them as Harlequins, and actually play them.
Examples of "Counts As"
- Autoguns count(s) as Lasguns in 40k Tabletop game (this may explain why Lasguns, despite being more powerful than most today's guns (maybe) are no more powerful than flashlights in 40k universe and that's because in tabletop game, a Lasgun's damage output is no greater than today's guns)
- Obelisks of Light from the Command and Conquer series count(s) as Lascannons in 40k Tabletop game
- Pile Bunkers like one that Slug Gunner uses in Metal Slug 5 counts as Power Fist(s) in 40k Tabletop game
- Tremor AGAS from Command and Conquer Generals: Zero Hour mod: Rise of the Reds counts as a Gatling Blaster in 40k Tabletop game
- Pyrovore as a Biovore
- Keyblades from Kingdom Hearts count(s) as Force Weapons in 40k Tabletop Game
- M134 Minigun counts as Assault Cannon
- Melody from Pokemon the Movie 2000 counts as Bonesinger
- Chapter Serfs count as Imperial Guard Conscripts (when they fight on the battlefield)
- Your entire army whenever you wish to play a Horus Heresy game
- Mark of Dave, for those of you who watch miniwargaming on YouTube
- shooting fireballs from hand count as shooting mind bullets
- RL instead of 40k example: Cartoon Physics are counts as Quantum/non-newtonian Physics
- Using an Anime giant robot model as a stand in for a titan.
- BM-13 Katyusha counts as a Manticore