Iago's character is complex, but in Act I, Scene I, where he describes his anger at not being made lieutenant by Othello, the main motivations for Iago's ways are revenge and anger; he has revenge for Cassio replacing him, and also anger that Othello gave Cassio the promotion instead of him. So it is obvious that Iago's manipulations are due to a basic desire to get his own back on those who hurt him, and to have what he wants, Iago's suggestion that Cassio and Desdemona are having an affair gets Othello to trust Iago and the position as his lieutenant in Act III, Scene III.
Being made lieutenant by Othello only satisfies his pride, he continues with his plan to discredit Desdemona and this shows that it is not enough for Iago to have what he believes is his, and he must do something to Othello for not giving him the job as lieutenant in the first place, by making Othello believe that Desdemona has had an affair and destroy her...
Iago is hardheaded. We see this in the way he uses this opportunity to hurt Cassio and Othello. When Iago realizes Cassio is fond of Desdemona, he immediately makes a plan to use this to make Othello suspect they are having an affair.
Although Iago appears to want Othello and Cassio to feel misery, he does not appear to have a specific plan that he has thought out, he continually uses insights he makes about Cassio, Othello, Roderigo and Desdemona to help his plan in his asides.
Iago's hard-headedness is also evident in his use of Roderigo. He uses this wealthy Venetian for money by promising to give the gifts he receives to Desdemona; he instead keeps them for his own purposes.
When Roderigo suspects this in Act V, he plots to have Roderigo kill Cassio because if Cassio wins, he also wins by having Roderigo killed in the process. In the end this part of the plan fails, so Iago kills Roderigo himself.
Iago is cunning when things get very hard, he nearly gets away with his plan; Othello kills Desdemona, he is made lieutenant, but Cassio even though he was wounded in the attack in Act V, lives and Iago is caught and exposed.
Iago’s weakness would be his wife Emilia, who although she receives threats and orders from Iago, revealed Iago's treachery by saying that Desdemona is innocent and explains how she found Desdemona's handkerchief, and how she passed it on to Iago... For this Iago shows ruthlessness by killing Emilia and running away, but is then later caught.
Iago appears to be using everyone else's weaknesses to his advantage, Iago does indeed have his own weaknesses, and this shows that no man is free from weaknesses.
Iago shows his weaknesses by his need to be made lieutenant, which reveals his own desire to reach a status he seems to need to be comfortable.
In Act II, Scene I, when Iago shows that he suspects Othello is having an affair with his wife as an incentive for manipulating Othello, he again shows his weakness, one which he uses with Othello against Desdemona.
Iago's great misjudgement is in his own wife. She stands up to him to defend Desdemona despite all the risks, unravelling Iago's manipulative web.
At the end of the play, Iago shows his difference from many of Shakespeare's villains in that he is still alive. Most villains in Shakespeare's plays die by being killed by those they done bad to in a theme of restoration of order. Iago on the other hand does not die; we can only guess that his future will be miserable...