Tyranids and Necrons. Tyranids are a swarm and hive mind race. They are meant to be an overpowering and overwhelming unstoppable force. Enabling this as a playable race would either make them overpowered and too easy to win with, or simply would require a nerf, therefor ruining the purpose of having Tyranids in the first place. The devs made a good decision in keeping them as a war-tide balance and as dungeon keepers. With Necrons, it's basically the same story as Tyranids, except they are not controlled by a single collective hive mind. They are individuals trapped inside machines, following a hierarchy of command. They also are not necessarily a swarm, but more like the Borg from Star Trek. They invade enmass, assimilate and destroy everything in their path, and move on to the next planetary or fleet-based objective. Even if they were for some reason ever to be made playable, they should not have the entire tree of units available (perhaps even restricted to basic infantry), and in-fact should also be restricted to computer or dev. designated targets and attacks. So they should not ever independently interfere with the dealings of the lesser races. It would simply be unfair and would cause a major imbalance to the game itself.
If you add IG as a playable faction you will seriously screw with the balance on the battlefield because of the popularity of space marines. It will also go from mainly infantry based combat to PS2 levels of vehicle combat. For IG to work right they would need to be f2p so they got the numbers to fight infantry combat and that may seriously ruin the Orks as a faction in game if they dont get enough f2p players.
I hate the Tau with a vile passion so i really don't want them in the game, but I realize thats probably a futile hope. However I really hope the Dark Eldar get in I want to play as a wrack. According the the Lore DE lack Psychic powers, but are four times more agile than there normal Eldar cousins so it could make for some interesting raiding combinations, Plus Body Modification, and torture chambers so much fun
i started playing TT FAR before any of the games came out that you mentioned, or i even knew it HAD games other than 40k and fantasy. But the TT rules are not the most accurate fluff representation. But you bring up good points. However, eldar have the same aspects as the ones you said DE have. They DO NOT need to be in vehicles--you will be able to mount bikes freely to get to places or use for combat, you will also obviously run a lot faster as eldar than other races, so i assume DE would be able to as well. There are also jet packs.
Tau work PERFECTLY as a playable faction...why do you hate them so much? i dont hate any of the factions--in fact i love every last one of them...
Oh! Maybe he just.. wait! Maybe he just has opinion. I bet it doesn't have anything to do with how playable it would be. No need to like them all..