First, can anything be a horde? if I have a team of of 5-6 marines on the verge of maxing their xp, would it be too crazy to have, say, a Horde of Chaos Marines?
Second, why the hulabaloo about not being able to roleplay a marine? Has no one played Dungeons and Dragons and played a paladin? Holy Avenger? HOLY BOLTER! You can have a personality and still be a zealot in service to the Emperor. No one whines about assassins who are basically mindwiped iirc, or preachers who sound like they eat, breathe, and crap the teachings of the Imperial cult.
No dramatic tension? Try running through a collapsing building firing a bolter one handed because you have a 6 year old girl cradled in one arm, protecting her from the slugga shells bouncing off your back! Do you deposit her somewhere and turn to meet the Orcs, who are certainly mocking you for running, or do you make sure she's safe at the cost of your pride?
As the CSM Codex and Horus Rising say: Marines are larger than life, in every aspect, physical, and mental. Huge muscles, bigger personalities. Think your co-worker gets pissed when you get a lighter workload, a Space Wolf is gonna fight you tooth and nail to achieve more glory? Imagine the verbal dueling a farseer and a Dark Angel Librarian could get into as they clash swords, talk about secrets!
I think people are having trouble getting out of tabletop mindsets, not because it's hard to, but because they don't think they CAN!
Doubt begets failure
HAVE FAITH IN THE EMPEROR!