I think its a matter of style whether East should open 1♥ or 1NT on this holding, 3rd in hand.  I prefere 1NT as I don't want to bid again, partner has already passed, but admit I might find myself in a lot of trouble.
South has a perfect TO double over a heart, no problems there.  But have you noticed what a good PASS North has made?  Who opened a weak 2♥ on this board and went down?

More on this later.

I can see East being in trouble on this hand.  It must be right to open 1NT - 13 points and a flat hand.  After three passes South is on lead, 4th best, a common lead against NTs will not do justice to South’s magnificent heart holding.  I think on this occasion you should lead the top - Partner won't expect too much then.  Some of you may ask why a heart and not a spade?  Hearts are your ‘long’ suit and spades will hopefully provide the entries needed to set the suit up.  Without any more information from the auction I don’t fancy anything else.

Partner should win the first trick with the ♥Ace and return the ♥10 won by declarers Queen.  East needs seven tricks to make his contract, two are Q & J ♥.  A♦ is three . . .   Ok, time to get the club suit going.  My bridge encyclopedia says lead small towards the King with this holding.  South wins the trick and plays a second heart clearing the suit.  East continues playing clubs – unlucky – they break 3-1 so North gains the lead and cashes his 4^th heart.  Unerringly he returns a spade, not leading away from the DK.  South now has 3 spade tricks and a fifth heart, 8 tricks in all, and 1NT -2.

What can go wrong for the defence?
South may decide to lead a spade originally, letting East win a trick with the King.  The timing is different; East should try to establish tricks in the club suit but still has two stoppers in hearts.  He now has the potential to make 3 clubs, 2 diamonds, 2 hearts and a spade.  Deal the hand out and try it if you don’t believe me.

The auction period is the time when you and your partner have a conversation.  Make the most of this and listen to what partner is saying.

You are South and open 1♠ after East passes.  West passes and partner responds 2♥.  2♠ from you and partner continues with 3♦.  Your turn next.

What do you know at this point?  Well lets start with the obvious, “a new suit at the 3-level is forcing” so you are certainly expected to bid again.  Partners hand pattern is at least 5 hearts and 4 diamonds, and does not contain 3 spades. 

Which suit do you have the best fit in? 

Do not be tempted to rebid spades again at this point.  Partner is already aware that you have 6 spades and has not supported them.  You, on the other hand, know that you have 3-card support for your partners 5-card major suit and should bid 3♥ now.  North has already shown 9+ points and will raise to game if he has a little more (ie a good 11 or 12).  Your second bid of 2♠ was a limit bid showing no more than an opening hand and a 6-card suit.

This puts North in the driving seat and in complete control.  He, and he alone,  will now decide if your side should be playing in part-score, game or slam.

Why play in the 3-5 fit rather than 6-2?

After you have drawn trumps, the long side suit will provide discards for you losers. 

On this hand NS can make 10 tricks in spades but 11 tricks in hearts for a top board and extra match-points. 

The principle is, when you are not playing in NTs, try to play in a major suit and select the suit with the flattest fit when you have the choice of hearts or spades.  The best fit is 4-4 in a major, second best is 5-3, but if you have stops in all the other suits you should also consider NTs as a final resting place.  Only play in minors with extreme shape.