still not quite done, but a better example

Block.php

<?php

/**
 * @file
 * Contains \Drupal\block\Plugin\Core\Entity\Block.
 */

namespace Drupal\block\Plugin\Core\Entity;

use Drupal\Core\Config\Entity\ConfigEntityBase;
use Drupal\Component\Annotation\Plugin;
use Drupal\Core\Annotation\Translation;
use Drupal\Component\Plugin\Exception\PluginException;
use Drupal\Core\Config\Entity\EmbeddedPluginConfigInterface;

class Block extends ConfigEntityBase implements EmbeddedPluginConfigInterface {
  // just demonstrating what it implements, so everything else is cut out
}

BlockInterface.php

<?php

/**
 * @file
 * Contains \Drupal\block\BlockInterface.
 */

namespace Drupal\block;

use Drupal\Component\Plugin\ContextAwarePluginInterface;
use Drupal\Component\Plugin\PluginInspectionInterface;
use Drupal\Core\Plugin\ConfigEntityAwarePluginInterface;

/**
 * Defines the required interface for all block plugins.
 *
 * @todo Add detailed documentation here explaining the block system's
 *   architecture and the relationships between the various objects, including
 *   brif references to the important components that are not coupled to the
 *   interface.
 *
 * @see \Drupal\block\BlockBase
 */
interface BlockInterface extends ContextAwarePluginInterface, PluginInspectionInterface, ConfigEntityAwarePluginInterface {
  // no need to see these here, just demonstrating what these implement
}

BlockManager.php

<?php

/**
 * Contains \Drupal\block\Plugin\Type\BlockManager.
 */

namespace Drupal\block\Plugin\Type;

use Drupal\Component\Plugin\PluginManagerBase;
use Drupal\Component\Plugin\Discovery\DerivativeDiscoveryDecorator;
use Drupal\Core\Cache\CacheBackendInterface;
use Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery;
use Drupal\Core\Plugin\Discovery\AlterDecorator;
use Drupal\Component\Plugin\Discovery\ProcessDecorator;
use Drupal\Core\Plugin\Discovery\CacheDecorator;
use Drupal\Component\Plugin\Factory\DefaultFactory;
use Drupal\Core\Plugin\Mapper\ConfigDrivenMapperInterface;
use Drupal\block\BlockInterface;
use Drupal\block\Plugin\Core\Entity\Block;

/**
 * Manages discovery and instantiation of block plugins.
 *
 * @todo Add documentation to this class.
 *
 * @see \Drupal\block\BlockInterface
 */
class BlockManager extends PluginManagerBase implements ConfigDrivenMapperInterface {

  protected $defaults = array(
    // 'semantic_wrapper' => 'div', @todo uncomment this once semantic markup wrappers are implemented
    'has_native_access_logic' => TRUE,
    // Indicates whether or not a block can be safely deferred to a subrequest.
    'subrequest_safe' => FALSE, // @todo it'd be great to default to TRUE here.
  );

  /**
   * Constructs a new \Drupal\block\Plugin\Type\BlockManager object.
   *
   * @param array $namespaces
   *   An array of paths keyed by it's corresponding namespaces.
   */
  public function __construct(array $namespaces) {
    $this->discovery = new AnnotatedClassDiscovery('block', 'block', $namespaces);
    $this->discovery = new DerivativeDiscoveryDecorator($this->discovery);
    $this->discovery = new ProcessDecorator($this->discovery, array($this, 'processDefinition'));
    $this->discovery = new AlterDecorator($this->discovery, 'block');
    $this->discovery = new CacheDecorator($this->discovery, 'block_plugins:' . language(LANGUAGE_TYPE_INTERFACE)->langcode, 'cache_block', CacheBackendInterface::CACHE_PERMANENT, array('block'));
  }

  /**
   * {@inheritdoc}
   * // @todo implement an interface that gets rid of the configuration array entirely.
   *
   * @return \Drupal\block\BlockInterface
   *   A configured BlockInterface instance.
   */
  public function createInstance($plugin_id, array $configuration = array(), Block $entity = NULL) {
    $plugin_class = DefaultFactory::getPluginClass($plugin_id, $this->discovery);
    return new $plugin_class($configuration, $plugin_id, $this->discovery, $entity);
  }

  /**
   * {@inheritdoc}
   *
   * @return \Drupal\block\BlockInterface
   *   A configured BlockInterface instance.
   */
  public function getInstanceFromConfigId($config_id, $plugin_id = '') {
    // Allow for different prefixes - e.g., custom vs. generic reusable blocks.
    $entity = config_load_entity_by_name($config_id);

    $plugin_id = $plugin_id ?: $entity->getPluginId();
    return $this->createInstance($plugin_id, array(), $entity);
  }

}

ConfigDrivenMapperInterface.php

<?php
/**
 * @file
 * Contains Drupal\Core\Plugin\Mapper\ConfigDrivenMapperInterface.
 */

namespace Drupal\Core\Plugin\Mapper;

use Drupal\Component\Plugin\Exception\PluginException;

/**
 * Defines an interface for retrieving plugins via a config id.
 *
 * For plugins that use the config system for their configuration CRUD, most
 * calling code will have only a config id, not a plugin id. The plugin id is
 * only stored in the configuration referenced by the config id.
 *
 * The most useful interface pattern for these cases is to simply allow calling
 * code to load the plugin by providing the config id.
 */
interface ConfigDrivenMapperInterface {
  /**
   * Gets a configured plugin instance from a configuration id.
   *
   * The calling code may optionally also specify a plugin id
   *
   * @param string $config_id
   *   The id of the configuration to load.
   *
   * @param string $plugin_id
   *   Optional. If provided, this plugin id will be used instead of whatever is
   *   specified by the loaded configuration.
   *
   * @return object
   *
   * @throws \InvalidArgumentException
   */
  public function getInstanceFromConfigId($config_id, $plugin_id = '');
}

ConfigEntityAwarePluginInterface.php

<?php
/**
 * @file
 * Contains Drupal\Core\Plugin\ConfigEntityAwarePluginInterface.
 */

namespace Drupal\Core\Plugin;

use Drupal\Core\Config\Entity\EmbeddedPluginConfigInterface;

/**
 * Interface for plugins that use ConfigEntity for their configuration storage.
 */
interface ConfigEntityAwarePluginInterface {
  /**
   * Sets the ConfigEntity for this plugin.
   *
   * @param EmbeddedPluginConfigInterface $config
   *   The ConfigEntity object.
   *
   * @return void
   */
  public function setConfig(EmbeddedPluginConfigInterface $config);

  /**
   * Returns the ConfigEntity for this plugin.
   *
   * @return EmbeddedPluginConfigInterface
   */
  public function getConfig();

  /**
   * Returns the string identifier of the instance id.
   *
   * This is typically, though not necessarily, the full name of the
   * configuration object that backs the plugin's ConfigEntity.
   *
   * @return string
   */
  public function getInstanceId();
}

EmbeddedPluginConfigInterface.php

<?php
/**
 * @file
 * Contains Drupal\Core\Config\Entity\EmbeddedPluginConfigInterface.
 */

namespace Drupal\Core\Config\Entity;

/**
 * Interface for ConfigEntities that are used as embedded config by plugins.
 */
interface EmbeddedPluginConfigInterface extends ConfigEntityInterface {
  /**
   * Returns the id of the plugin in which this configuration is embedded.
   *
   * @return string
   */
  public function getPluginId();

  /**
   * Sets the id of the plugin in which this configuration is embedded.
   *
   * @param string $plugin_id
   *   The id of the plugin.
   *
   * @return void
   */
  public function setPluginId($plugin_id);
}