1. They're both "country" folks. Now, by modern American standards, this might imply poverty or poor manners, but during Austen's time, it simply meant that they were not part of the aristocratic elite (like the Bingleys and Darcys who own several homes and divide their time between London and the countryside).
2. Both Charlotte and Elizabeth are part of families which rely upon the daughters to marry well. Austen does not supply as much information about the Lucases, but the reader knows that because Mr. Bennett has no sons, his "estate" will go to a more distant male hier, and the girls must marry well to help provide for their parents. Charlotte is in the same situation and marries Mr. Collins (Elizabeth's distant cousin) in order to provide "independence" for herself.
i must inform you that socially charlotte's lucas's family was a bit high-ranked than the bennets. sir william lucas attained knighthood. but,if you consider their general behaviour,say,mrs. bennet's vulgarity and mrs. lucas and her husband's cheap attitude(when they came running to the bennet's doors carrying the news of collins' proposal to charlotte)-you can notice their similarity... but i must add that , mr. bennet was far more witty and intelligent (although to some extent sarcastic) than mr. lucas.