I'd recommend just saying you either aren't able to play with them for some nebulous reason or say that you don't feel your playstyle matches the group. Don't make a big deal about it, don't allow yourself to be dragged into an exposition that will cause players to become defensive and argue with you while you're walking out the door. Being on the way out, your decision made, unless you wish to give them the opportunity to either a) disprove your thoughts as you go or b) disprove your thoughts/make amends so that you will stay, I'd keep it as brief as possible. Blaming oneself is hardly the best option, but I highly doubt your average gamer welcomes the drama or the commentary on what you don't like about them... and unless your goal is to offer them exactly that, I'd skip it if possible.
If you don't, you'll still be the guy leaving, and you'll be leaving them to come to the conclusion that, regardless of how rational you were in your opinions, you ultimately abandoned the group without either offering them the opportunity to alter their styles or that you got your final jabs in, many of which that are quite inaccurate in hindsight for X, Y, and Z reasons discussed repeatedly over the next few sessions, and that you're mean and they're possibly better off without you.
And of course, I could be absolutely wrong about everything stated in the second paragraph... and that's part of my point. You can control, in leaving, exactly what they're possibly right or wrong in thinking, but you can't control their thoughts being right or wrong (and i say this in relation to your thoughts, not in a "absolute capitaltee Truth" way). The moment you open the floor to discourse or plant the seeds of contention in the group, it's all thrown to the wind. They might just accept your comments with class, they might blow up, they might heckle you for the next few days as they consider your wisdom and comments little more than cowardly "I'm leaving and oh as the door hitsmyassyouguysarebadforthefollowingreasonandalthoughIdidn'twant (gasp) totellyouanythingthatwouldupsetyouIdidjustsohappento (gasp) createthislistjustontheoffchanceyouwerecuriouswhyiwasleaving." comments said by someone who could neither handle the social implications of your group's different playing styles or personalities nor deal with the potential backlash of such statements. Again, neither of which may be the case.
So, to make a repetitive stance concise (and cookies to you if you are a person who reads the bottom line first), just bow out gracefully and if necessary chalk it up to something impersonal like "I don't think my playstyle matches everyone elses, but thank you for the good times anyway." Go beyond that at your own peril.