What makes a good duke?

• Stats: There are two stats you want to look out for when picking your middle-managers: Stewardship and Martial. The first increases the amount of income they get, which allows them to upgrade their holdings and increases the taxes they pay to you (if you tax that kind of vassal). Martial increases the amount of troops they provide.
• Traits: You should pick people with traits which give them an opinion-bonus with you, because the better their opinion, the more troops and taxes you get from them and the less likely they are to cause you any trouble. Note that there are also some traits which cause hidden changes to AI behaviour. Traits like "Deceitful" make them more likely to take part in plots. Traits like "Wroth" make them less "rational" which causes them to make stupid decisions.
• Content vs. Ambitious: These two traits are so important that they deserve special mention. The content trait comes with some slight attribute penalties, but is the easiest to handle due to their +50 opinion bonus towards their ruler. That makes content people usually the first choice for leadership positions. Ambitious is a double-edged sword. On the one hand, Ambitious rulers will always look for ways to expand their, and in extension your, realm. This makes them very likely to declare wars on outsiders. So when they have enough potential enemies outside the realm to declare war on, they will do so, which can be a great asset. But on the other hand, when they have nothing to conquer, they will look into how to gain more power inside the realm, which means they will get the hefty "ambitious" opinion penalty towards you and try everything to stir up trouble.
• Do not let them become too powerful: Avoid giving single vassals too many counties. Even when the current one might be quite easy to handle, think ahead for a few generations. It's hard to depower a large duchy once formed, and you don't know how their grand-grand-children will turn out. A future superduke who hates your future character can become a great pain in the ass. So before you make someone a duke, check if they aren't going to inherit another one.
• (Conclave DLC) usefulness as council member: With the conclave DLC, dukes will want to be on your council. That means you should look for dukes which have one exceptionally high stat so they are actually useful in that position.
• Remember to look at their kids: Keep in mind that when they kick the bucket you will have to deal with their heir. When their heir is complicated or a moron, that's a deal-breaker. Unless, of course, you can make an accident happen to them and their younger siblings are more reasonable.
• Your next character:
  • Giving your heir under gravelkind or (...)geniture some of their inheritance during your lifetime allows them to build up their power-base before you die. Being a ruler makes them grow stats more quickly and it shortens the "new ruler" opinion penalty for their direct vassals. It also removes the "unlanded heir" penalty. Unfortunately some restrictions apply to giving titles to heir-apparents which I don't fully understood yet.
  • When you are ruling an elective realm, consider giving your duchies only to people from your own dynasty. Duchies are usually (always?) hereditary titles, so any future dukes will very likely also be of your dynasty. Any duke is eligible for becoming the next king, and when they are from your dynasty, you can continue playing with them. This frees you from the headache of always persuading them to vote for your son so you don't get a game over. As a king your vote for the next king is very influential, so it gives you the option to pick your next character from your dukes, which is very useful indeed.

Their opinion of you, as you mentioned is an important factor. Other traits that I'd look for when choosing a duke are their:

• Skills: Their overall stats will have different effects on the territory, for example a duke with a high stewardship will lead to a higher amount of gold, which will also lead to a higher amount of gold for you. The same applies to martial and learning. A duke with a high intrigue skill can be a good thing if you utilize them as your spy master but they can always use that skill against you with assassination plots.

• Traits: Different traits make each character behave differently. A duke with the ambitious trait and a high martial could potential start a faction and have enough men to make a serious threat. I recommend looking for a candidate with the content trait. This trait automatically gives your candidate a +50 opinion towards his liege and makes him ineligible for most plots. Trait list

• Leadership: Dukes can be potential candidates for leaders during combat. If you're regularly at war or want another commander for future battles, then choosing someone who will be a good fighter can really help in a battle, especially if you're outnumbered. Refer to the link above for a full list of leadership traits.

Those are some of the key factors that I look for when choosing a duke, always remember that they are your closest allies, and potential enemies.