Convention: a way in which something is usually done, especially within a particular area or activity.

Love is prevalent in every Shakespearean comedy. Orsino seems obsessed with the idea of love. Many characters spend a lot of their time falling in love even when we as the audience know they are being deceived.

Viola wants to marry Duke Orsino, but she can't because Orsino is in lust with Olivia. Plus, Orsino thinks Viola is a boy, "Cesario."

Also, Olivia wants "Cesario," but she can't have "him" because "Cesario" is Viola, who loves Orsino.

Malvolio believes that Olivia loves him... but it's a trick set by Maria.

Everyone's in love, but things are a mess. That's the "obstacle" part.

Here's how love "overcomes" obstacles: When Sebastian shows up, Olivia has no problem transferring her lust for "Cesario" to Viola's look-alike brother. (Yeah, it's ridiculous but so what?) Sebastian's a bachelor so, hey, why not marry Olivia? Now that Viola knows her brother's alive, she can reveal her identity, which is a good thing because now the Duke can marry her. That was easy.

There's plenty of humor in the play but at the same time Twelfth Night addresses complex controversial issues – like sex, gender, love, social ambition. It just does so using humour, which can be the most effective way to address a heavy topic.

You could compare it to Jon Stewart's treatment of politics in The Daily Show. It's silly and light but also takes on complicated issues.

The element of the improbable or supernatural can mean magic and fairies in some Shakespearean comedies. In Twelfth Night, the miraculous is demonstrated right from the beginning when Viola and Sebastian both survive the shipwreck and end up on the same island. Big coincidence!

There is also the fact that Viola is able to convince everyone she is a man for most of the play. Mistaken identities often occur in Shakespearean comedies.

Because there are multiple strands of action competing for our attention, we never get too deeply involved. It is easier to laugh when you are not too deeply invested.

Instead we are intellectually alert and uncommitted, able to compare and contrast different experiences.

Even though the play taunts us with the possibility of some unconventional couplings (like "Cesario" might be mauled by Olivia), Twelfth Night, ends in the marriage and/or heterosexual pairings. This is a way for order to be restored to the Elizabethan universe (they were very big on nuptials). By the end, Olivia hooks up with Sebastian in a secret ceremony, Toby and Maria get hitched (off-stage) and Viola and Orsino get engaged.
Shakespearean comedies , like many comedies today, tend to affirm the societal norms they mock.

Lots of other subtle elements to comedy too.

Mistaken identity - think Viola, Cesario, Sebastian

The bed swap - less 'bed' in 12th Night, but think of Olivio marrying Sebastian, believing him to be Cesario.

Playing with taboo - homosexuality, gender boundaries, cross-dressing

Slapstick - physical comedy with Malvolio's ridiculous dressing up

Comic dialogue - look out for heavy use of pun, misunderstandings, confusions, exaggeration (think Duke Orsino and love...), funny contrasts (the serious Malvolio dressed ridiculously)