I'm not really good with imagery, so I'm not sure if I could live up to that standard of post.

Couldn't there just be a problem with the speech center of their brain, though, where complex concepts can't really be quantified?  It would be like having a concept in your head, but not being able to express it.  We could still understand concepts, but would have trouble relaying that info to others.

It would also explain the limit on word usage.  We could say that expressing these words as speech causes misfired in the brain.  They slowly grow in strength, until around 10k words, at which point it causes a seizure that damages autonomic brain function, stopping the heart.

Of course the biggest problem with that is that emotions would be one of the things they could express the best.  Great apes, for example, have underdeveloped speach centers compared to us, but have no problem expressing needs through sign language, or emotions through body language.