I think that in many ways, the thematic implications from Simon's drama are quite profound. One of the major themes relates the dynamic nature of relationships. The relationship between Felix and Oscar, one that is "consummated" of sorts when they move in together, fundamentally fails because neither one is able to adapt to the ways of the other. Marriage and all relationships are shown to fail when there is only dual monologues, instead of a legitimate dialogue present. Both Oscar and Felix never adapt to the other. Rather, they believe that by simply "being" all will be well. In this light, Simon is making a point that all relationships can only work when there is an acknowledgement of the need to integrate aspects of the other into one's own state of being. It is through this whereby relationships can work. Failure to do so will result in a relationship failure.
Another real interesting theme that comes out of the work is how both men learn to be better people. Both men learn that conversation and dialogue are necessary for any relationship to work. Though their own relationship might not have worked out in the manner each expected, there is reason to believe that both have changed as a result of their experience together. Oscar has become a better husband in terms of paying his alimony and wishing to reestablish some semblance of communication with his ex- wife and son. This is a testament to the communication skills he had to grasp through his time with Felix. In living with another man, Oscar might have become better with another woman. For Felix, he has changed, when he tells Murray to tell his wife that he "is not the same man." What exactly this is is not certain, but there is a spirit of adventure and excitement that is in Felix now that was not at the start of the play. Presumably, his time with another man has made him a happier one. In this, change is evident in both, demonstrating that men have the power and capacity to change. This is a very interesting and unique theme both in the historical context of the play and in the modern setting, as well.