[autumnstwilight]

In my experience, the words that carry the most information in a sentence are the less common ones. Here's what understanding 80% of a sentence is like:

"I went to the sdjfkdsh and got a new ghjsakgfh."

The missing words could be "dealership" and "truck" or "embassy" and "passport" or quite a lot of other pairs that change the topic entirely, so reading or listening to something with 80% understanding generally requires a dictionary in one hand to get you up to a reasonable level of comprehension. That said, I personally think language learning is enjoyable and rewarding, and tackling the most common word list is a good first step.