The audience is probably feeling many emotions and the expectations could take many avenues at this point. The audience is hopeful at the end of Act I Scene 5 that Romeo and Juliet will defy fate and end up as a happy couple. Even though they know of each other's families at the end of Scene 5, they are willing to defy anyone or anything to be together. This makes the audience feel that despite the foreshadowing in the Prologue using the words: "misadventured" and "death-marked," to name a few, they will still beat the odds.
Romeo's speech at the end of Scene 4, as he approaches the Capulet's feast, indicates that he sees things going terribly wrong and that this will lead to his eventual demise.
Tybalt vows to kill Romeo and get his revenge because he is a Montague and he dared to enter the Capulet's feast. The audience will see Tybalt as a serious threat to the couple's happiness. He says, "I will withdraw, but this intrusion shall, Now seeming sweet, convert to bitter gall."
I think that even though we know Romeo and Juliet is a tragedy, we are also hopeful, as an audience, that love will prevail and will overcome the obstacles. I also think the audience feels that they are on the edge, fearing the worst, but hoping for the best.
Reference: The Language and Literature Book by McDougal Littell
The audience is likely to expect several things from the scene. First and most simply trouble, and that means action—drama! Romeo is where he is not wanted, and is likely to be seen as an intruder. Second, since the prologue indicates that these are star-crossed lovers, some elements of the audience may be expecting if not love, at least for the hook to be set to catch the two lovebirds. Third, given how Lord Capulet speaks at the start, they should expect some humor, and fourth, they should expect a kind of lively excitement from the party itself.