This YouTube shortcode for WordPress automatically grabs a video's thumbnail, stores it locally and presents the video as a clickable div with a responsive and lazy loading background. I found this useful after having paid a heavy loadtime on my archive of favourite YouTubes.

youtube-jquery

// Build lazy loading function - visible that is...
(function ($) {
	$.fn.visible = function (partial, hidden) {
		var $t = $(this).eq(0),
			t = $t.get(0),
			$w = $(window),
			viewTop = $w.scrollTop(),
			viewBottom = viewTop + $w.height(),
			_top = $t.offset().top,
			_bottom = _top + $t.height(),
			compareTop = partial === true ? _bottom : _top,
			compareBottom = partial === true ? _top : _bottom,
			clientSize = hidden === true ? t.offsetWidth * t.offsetHeight : true;
		return !!clientSize && ((compareBottom <= viewBottom) && (compareTop >= viewTop));
	};
})(jQuery);

// Run the lazy loading
$(window).on("load resize scroll", function (e) {
	$('.lazy-wrapper:not(.loaded)').each(function () {
		if ($(this).visible(true)) {
			$(this).css('background-image', 'url(' + $(this).data(original) + ')');
			$(this).addClass('loaded');
		}
	});
});

// Load YouTube on user click
$(document).ready(function($) {
  $('.video-btn').on('click', function() {
    var self = $(this).parent();
    if ( self.hasClass('lazy-youtube') ) {
      var ID = $(self).attr('data-id');
      var HOME = '<?php home_url( "/" ) ?>';
      var YOUTUBE = '<div id="wrap-video"><iframe type="text/html" class="video-embed lazy-video" width="640" height="360" src="http://www.youtube.com/embed/' + ID + '?modestbranding=1&autoplay=1&autohide=1&controls=2&theme=dark&showinfo=0&rel=0&enablejsapi=1&origin=' + HOME + '" frameborder="0" seamless webkitAllowFullScreen mozallowfullscreen allowFullScreen></iframe></div>';
      self.html(YOUTUBE);
    }
  });
});

youtube-shortcode

<?
/**
* Get YouTube ID by URL
*/
function gr_youtube_id($url) {
	$pattern = '#^(?:https?://)?(?:www\.)?(?:youtu\.be/|youtube\.com(?:/embed/|/v/|/watch\?v=|/watch\?.+&v=))([\w-]{11})(?:.+)?$#x';
	preg_match($pattern, $url, $matches);
	return (isset($matches[1])) ? $matches[1] : false;
}

/**
* Build fragment caching
*/
function gr_cache($key, $time = 3600, $function) {
	if (
		is_user_logged_in()
	) {
		call_user_func($function);
		return;
	}
	$key    = apply_filters('fragment_cache_prefix','gr_cache_'). $key;
	$output = get_transient( $key );
	if ( empty( $output ) ) {
		ob_start();
		call_user_func( $function );
		$output = ob_get_clean();
		set_transient( $key, $output, $time );
	}
	echo $output;
}

/**
* Get YouTube thumb by URL and save it locally. Than pass it as data-variables for lazy loading.
*/
function gr_youtube_lazy( $url = NULL ) {
	$id                     = gr_youtube_id( $url );
	global $url, $id;
	gr_cache('youtube_'. $id, 86400, function() {	
		$upload         = wp_upload_dir();
		$path           = $upload['basedir'].'/cache';
		$thumb_path     = $upload['basedir'].'/cache/youtube-'. $id .'.jpg';
		$thumb_path_uri = $upload['baseurl'].'/cache/youtube-'. $id .'.jpg';
		if ( !file_exists($path) ) {
			mkdir($path, 0777, true);
		}
		if ( file_exists( $thumb_path ) ) {
			$thumb  = $thumb_path_uri;
		} else {
			$json   = json_decode( file_get_contents('http://gdata.youtube.com/feeds/api/videos/'. $id .'?v=2&alt=jsonc' ) );
			$thumb  = $json->data->thumbnail->hqDefault;
			if ( $thumb ) {
				$img_path = file_get_contents( $thumb );
				$img_save = file_put_contents( $thumb_path, $img_path );
			}
		}
		$video  = '';
		$video .= '<div';
		$video .= ' class="lazy-wrapper video-wrapper lazy-youtube ratio-wrapper ratio-16x9"';
		$video .= ' data-original="'.    $thumb .'"';
		$video .= ' data-id="'.          $id .'"';
		$video .= '>';
		$video .= '<div class="video-btn"></div>';
		$video .= '<div class="ratio-expander"></div>';
		$video .= '</div>';
		return $video;
	});
}

/**
* Build shortcode
*/
function gr_youtube_shortcode($atts, $content = NULL) {
	extract( shortcode_atts( array(
		'url' => ''
	), $atts ) );
	ob_start();
	if ( $url ) {
		echo gr_youtube_lazy( $url );
	}
	$content = ob_get_contents();
	ob_end_clean();
	return $content;
}
add_shortcode('youtube', 'gr_youtube_shortcode');

youtube-style

.video-wrapper{
	padding-top:56.25%;
	position:relative;
	background-color:#000;
	-webkit-background-size:cover;
	   -moz-background-size:cover;
	     -o-background-size:cover;
	        background-size:cover;
	background-position:center;
	background-repeat:no-repeat;
}
.video-wrapper .video-btn{
	position:absolute;
	z-index:99;
	left:50%;
	top:50%;
	margin:-40px 0 0 -50px;
	width:100px;
	height:80px;
	background:url('../../images/video.png') top center no-repeat;
}
.video-wrapper .video-btn:hover{
	background-position:bottom;
	cursor:pointer;
}
.expander,
.video-embed{
	position:absolute;
	left:0;
	top:0;
	width:100%;
	height:100%;
}