Some of you may have seen this video on Mile High Report, but it's worth promoting here because it's such an awesome video detailed explanation by Wade Phillips of his 3-4 techniques:
He explains that you coach the players differently based on what their skill set is. Every player at every position has his assignment, but they might line up differently or play differently depending on their abilities. For instance, a faster SSLB will line up a shade further outside the TE if he's a faster player, because he can get an angle to get round the TE and get to the QB in pass-defense, but a slower stronger guy might line up tighter on the TE because he doesn't have the same speed. That faster guy can recover quicker and get back on the TE even though he's out wider because of his speed so it allows him to take that wider angle.
Their assignments are the same but how they play can be totally different because you adjust the scheme to their particular strengths and weaknesses. I thought this was an excellent point.
Seeing this reminds me why Wade Phillips has been such a successful defensive coach, he coaches to his players, he doesn't try and force all his players into a mold. He says the essence of coaching is knowing the strengths and weaknesses of each player and coaching him accordingly.