One kind of opaque thought is an idea that is perceived in a general but not a specific sense. Ordinary conversation and conventional wisdom are full of examples of this:
Athletic promotes good citizenship.
Older people are more conservative than younger people.
A limp handshake is a sign of a weak character.
The point is not whether these observations are accurate or not. In fact, each one may be accurate in some instances and not in others. People run into trouble if they try to write about things like these without relating to their own experience and knowledge. The sentences will not unclear in the sense that they will be misunderstood. They will be unclear in the sense that the reader will think he understand, but in actuality he will take away no real understanding from such general ideas unless the writer is able to relate them to specifics. See the following example:
There were many types of people in the room.
A sentence like this does not convey much much. What does the writer mean by "different types"--social class, profession, nationality, religion, physical size, age, political orientation? If the writer uses more specific language, the reader can get a more complete idea of the people in the room:
An overdressed, middle-aged lady paced nervously in one corner of the room. A barefoot youth sprawled over a chair and stared at the ceiling. A thin, unkempt woman sat opposite me, smiling and rolling her eyes whenever I glanced her way.