Why doesn't everyone use Avada Kedavra?

• Why Death Eaters didn't use AK was already discussed here: Why don't Death Eaters always use Avada Kedavra?

  Also, while not mentioned in canon, another reason Bellatrix specifically didn't use it in a duel with Molly was underestimating the opponent. She was not exactly thinking rationally.

• As far as why those opposing Death Eaters (let's notionally call them Good People) didn't use it:

  1. You need to be a strong magic user for AK to work. Fake-Moody explained that in GoF:

     "Avada Kedavra's a curse that needs a powerful bit of magic behind it — you could all get your wands out and point them at me and say the words, and I doubt I'd get so much as a nosebleed."

  2. Legal repercussions.

     Avada Kedavra is illegal. Aurors needed explicit authorization in the First Wizarding War to use the three Unforgivable Curses.

  3. They are "moral".

     They don't wish to kill.

  4. They obviously aren't trained in using it.

     Especially housewife Molly Weasley, since your question explicitly singled her out.

  5. Specific to Molly's duel - she wouldn't want to use Avada Kedavra since a miss would be risking to kill an innocent bystander, even if she knew how to cast Avada Kedavra.

There are a variety of reasons. Avada Kedavra isn't the bread-and-butter spell it might seem.

• First and foremost, as mentioned elsewhere, Avada Kedavra is a very difficult spell to cast effectively. As Bellatrix says, and as Harry notes with the Imperius Curse later on, "you have to mean it." The fact that Voldemort can sling around Killing Curses without any trouble is a sign of his prodigious magical abilities and of his single-minded desire to kill his opponents. In the midst of a pitched duel, there's not much difference between killing or disabling your opponent with a curse like Dolohov's, using Avada Kedavra or even Stunning them, although in the latter case they might be revived by their compatriots. Thus, since the Killing Curse is so much harder to cast, most combatants will opt for a different spell. We see that even most Death Eaters don't use this curse continually. Only Bellatrix and of course Voldemort seem to be able to keep it up.

• Avada Kedavra can't be blocked by any magical shield or counterspell, but it can be blocked by interposing an object between oneself and the attack. As shown by Dumbledore, this is a highly effective method. Spells that move objects or potentially even conjure them can be an effective defense in the hands of a skilled magic user. Some spells other than Avada Kedavra may simply ignore the presence of objects or produce effects that go around them.

• Although it's not elaborated on, most offensive spells seem to travel at finite speeds. Avada Kedavra in particular is slow enough to dodge. This, by the way, is one reason it wouldn't have immediately ended the duel. In fact, the odds are good that they were both casting Avada Kedavra; Molly Weasley's spell was simply the first that hit. Physical ability is in fact rather important in dueling, all else being equal. It's no coincidence that Harry, with his excellent Quidditch reflexes, is so good at combat magic. With this in mind, there may well be spells that travel faster than Avada Kedavra but are more easily blocked, creating a trade-off.

• Some spells can affect areas, whereas the Killing Curse affects a single target. This makes them potentially much more useful in an asymmetric fight; even if the opponents have a better chance of blocking them, they could take out multiple opponents at once, whereas Avada Kedavra would take out only one, leaving one open to a counterattack. An example of this would be Grindelwald's Protego Horribilium from the second Fantastic Beast film: it let him take out many targets at once. But this tradeoff isn't limited to fancy spells that only very powerful witches and wizards can use: Reductor, which blows things up, is a more common example.

• If your opponent is too weak or not inclined to use Avada Kedavra, but they're dodging your attacks and firing back some spells, you may be better served by casting a defensive spell instead. For instance, Voldemort had to conjure up a shield against one of Dumbledore's attacks.

• The Killing Curse is not proof against Apparition, which allows one both to evade it or simply to escape. Thus, in some circumstances it might be best to cast spells that limit Apparition first. With the capability of killing people before they successfully Disapparate, Grindelwald's spell is an example of this as well.