What to lead in partners suit?

Last deal we learnt that making lead-directing bids is hugely beneficial for the subsequent defence. Say partner has bid a suit, and you are (naturally) going to lead it. Which card is correct? Here are the key rules, in decreasing priority order:

What happened
Believing (erroneously) that it was always correct to lead top of partner’s suit, West led ♥ K against South’s 4 ♠. And now there was no defence to defeat the game. A second heart was led to East’s ♥ A, and East switched to ♦ Q. Declarer won ♦ A, drew trumps in three rounds finishing in hand, then cashed the promoted ♥ Q discarding ♦ 4, and followed by cashing ♦ K and trumping ♦ 6 with dummy’s last trump. Just a club was lost from here - 10 tricks and game made. Even if West had switched to ♦ 2 at Trick Two, declarer would have succeeded, winning ♦ A, drawing trumps finishing in dummy, and leading towards ♥ Q, thus promoting the card for a diamond discard.

What should have happened
West should have led ♥ 2, not ♥ K (key play). This shows his liking for the suit (lead low for like), but retains ♥ K (in this instance to beat ♥ Q). East wins ♥ A, and returns ♥ J, covered (say) by ♥ Q and ♥ K (or ♥ J will win the trick). With ♥ Q not promoted, declarer must lose a diamond and a club in the fullness of time. Down one.

If you remember one thing...
Do not necessarily lead top of partner’s suit - only do so when you have the ace, a doubleton, or a sequence.