As I've become more involved in meetings at work, I've noticed a myself speaking like you describe. It's as though I've trained my hind brain on the topics being discussed and can just let it pattern match the conversation to generate and issue a response. Problem is I often don't even remember what I said, much less am able to build upon it when questioned.

The change has probably come about due to the pacing of conversation. I remember that before I'd build a statement in my head, test phrasing a couple different ways, check that it made sense in context, then say it. That's quite a bit slower, though, and often by then the discussion has mutated such that the constructed statement no longer fits.

On those rare occasions of deep, thoughtful conversation is when I notice the difference. Thoughtless speaking seems to be something I slip into, while thoughtful speaking is very deliberate - I have to consciously stop and force myself to actually focus on and consider the topic sometimes, especially if it's been a while.

Like maxhudson suggested, writing seems to me to most closely match the process of thoughtful speaking. Even if the first pass is on autopilot, you can go back and reevaluate, analyze what you wrote.