This is a brief, precised addition to the above answer.
Some important themes of The Way of The World are:
1) Lifestyle of the 18th century Restoration society, problems in both personal & social behaviors, manners & customs
2) Feminine vulnerability
3) Country vs. urban life
4) True love (between Mirabell & Millamant) vs. fake love (between Marwood & Fainall)
5) True wit vs. false wit (Mirabell vs. Petulant, Witwood, Sir Willful Witwood)
You have to connect these themes yourself with the plot, and thus your own critical analysis will be built up.
Adultery, pretentiousness- all these were common features of that age especially among the aristocratic men and women. These traits are connected with the first point.
The meaning of the expression "the way of the world" literally means 'the way people behave or conduct themselves' in this world. However, in the Restoration times which was notorious for its promiscuity and loose morals the expression "the way of the world" connoted adultery.
Adultery is the most important theme of Congreve's play "The Way of the World," and it is underscored by using that expression as the title of the play itself.
The expression "the way of the world" occurs thrice in the play:
Firstly, at the end of Act 3 Mrs. Marwood reveals to her lover Fainall the details of the conversation between his wife Mrs.Fainall and Foible which she overheard when she was hiding in Lady Wishfort's closet. Fainall becomes acquainted with the bitter truth that his friend Mirabell and his wife Mrs. Fainall had been former lovers and that Mirabell had got him married to Mrs.Fainall to use him as a shield in case Mrs. Fainall were to become pregnant. Fainall is shocked to learn of the betrayal of both his friend and his wife and expresses his resentment thus:
Fain: And I, it seems, am a husband, a rank-husband; and my wife a very errant, rank-wife,—all in the way of the world.
Secondly, in Act 5 Mincing the servant steps forward to testify that she and Foible had seen Mrs.Marwood and Fainall in a sexually compromising situation, at once Fainall very boldly remarks:
Fain: If it must all come out, why let ’em know it, ’tis but the way of the world.
Fainall defends himself saying that he couldn't care if he is exposed as an adulterer because he knows the truth that his wife is also an adulteress. Adultery, according to Fainall, is too common a practice for anyone to complain about.
Thirdly, again in Act 5 Mirabell taunts Fainall by remarking,
Mira: Even so, sir, ’tis the way of the world, sir; of the widows of the world.
Mirabell snubs Fainall by revealing to him that his wife, a former widow, Mrs.Fainall who was actually his secret lover has been wise enough to trust him with her share of her property and that every thing had been recorded precisely in a legal document and that he had no rights over her property.
Mrs.Fainall even as she was committing adultery had been shrewd enough to protect her financial interests unlike her cuckolded husband Fainall.