I think what impresses Huck the most about this house is how wealthy the people who live there must be. It is a very nice house with lots of rich stuff in it.
When Huck first goes in, there are people pointing guns at him, but he still notices how nice it is. He says that the parlor they go into is big and it has a new rug. If he notices how nice it is in that kind of situation, it must really be nice.
Then, the next morning, he thinks a lot about how nice the house is and how it is just as nice (and nicer) than houses he has seen in towns even though it is a house out in the country.
So he is clearly impressed by how nice and rich the house is.
You can find more details in Chapter 17.
Briefly,what impresses Huck most about the Grangerfords' home is the huge of the house ,the furniture,pictures...etc...everything. but the most important Question is Why? Simply because he didn't see a mansion like these in his life and he compare it with the houses in town.