How is Act I an effective opening to Othello?
I suppose it might be possible to argue that the play begins in rather a strange place: two characters stroll on the stage, mid-conversation, and we have to struggle as an audience to work out what it is they are actually talking about. Yet if we focus on their words and the nature of what Iago is telling Roderigo, we can see that this opening to the play is very effective because it hooks the audience and makes us want to see what will happen next after the initial exposition of the central problem, which is Cassio's promotion instead of Iago.
Iago's response to this and to his hatred of "the Moor" is fascinating. He explains to Roderigo that he will pretend service whilst plotting something else entirely:
In following him I follow but myself:
Heaven is my judge, not I for love and duty
But seeming so, for my peculiar end,
For when my outward action doth demonstate
The native act and figure of my heart
In complement extern, 'tis not long after
But I will wear my heart upon my sleeve
For daws to peck at: I am not what I am.
This is so intriguing to the audience because it gives us a key aspect of Iago's character that fascinates us and makes us curious about how this quality of his will be developed. As always with the best of villains, Iago's character is one that fascinates and attracts us even as it repels with equal measure. Such a focus on Iago's motivation at the opening of the play makes it effective.