Diego Costa has scored seven goals in just six Liga matches. He was Atleti's second weapon last season when Falcao was still around, but he's now Simeone's best offensive threat even after signing David Villa away from Barcelona. Costa has the full package: he fights -sometimes way too much-, he knows how to move, he's tall and he can even play on the right wing.
If Costa wasn't enough, Atlético Madrid's other striker is David Villa. Considering that Simeone usually plays with a 4-4-2 system, Villa is the striker trying to create some moves and find open space to finish plays. Villa is not the player he was, that's obvious, but he's still a good finisher and Real's defenders should be afraid of what his right foot can still do.
As we stated above, Atlético is a really solid team with a 4-4-2 system. Gabi and Suárez usually are the team's midfielders. Both of them are willing to fight for every ball and it will be very difficult for Real Madrid to find the space between the midfield and the defense with them around. Koke and Arda Turan will likely be Simeone's wingers. Koke is a very similar player to Sami Khedira. He usually plays on the right wing, but his role is very close to what Mourinho wanted to do with the German player. Arda Turan has not been playing really well brilliantly, but he has the quality to harm Real Madrid's defensive line. Atlético Madrid need him to play better and he will surely be an important player for Simeone. Raúl García should wait on the bench, but he's also the kind of player Simeone loves. García knows how to move and finish second chances, and he's also really powerful.
It will be hard for Real Madrid to win the physical battle against Atlético Madrid's midfield. But Simeone usually likes to give the ball to the opposition and enjoy some counterattacks. Therefore, Modric and Isco's performance will be hugely important for the game.
Atleti's defensive line isn't great, but they have only conceded five goals. Juanfran is still playing as a right back -he was a right winger for Osasuna-, and Filipe Luis will always try to help his teammates offensively. Godín and Miranda are Atleti's center backs. Not brilliant but not bad either. If Benzema and Cristiano Ronaldo move well enough, Miranda and Godín should struggle.
Thibaut Courtois is still Atleti's goalkeeper. He's spending his third season on loan from Chelsea, and at age 21 he's already one of the best goalkeepers in the Liga BBVA. Chelsea would be crazy to sell him, so Atlético Madrid might have to find a replacement sooner rather than later.
Considering that the game is played at the Bernabéu, Real Madrid should expect a hard-working Atlético Madrid team trying to steal the ball in dangerous zones and then finding the open space with Costa and Villa. It would be surprising to see Simeone's team dominating and controlling the tempo of the game, but that doesn't mean they're less dangerous.