I feel like a successful mechanic should be satisfying to the players.  I know that sounds all so vague.  But at the end of the day, there is no perfect formula for a successful mechanic.  A mechanic is successful if it makes the player feel good.  I have noticed a few things that goes into the definition of a “successful” mechanics.  While these are not hard rules, they have helped shape my designs.

• A successful mechanic should fit into the game.
  An example of a mechanic that does this pretty well is the suicide move from Samurai Showdown 4.  Players can sacrifice their own life and forfeit the current round and start the next round with a full super meter (POW guage).  The theme of a suicide or seppuku fits well in a game about samurai fighting and it adds a level of strategy in the meter building game which is often crucial in a fighting game.    
• A successful mechanic has a purpose and can deliver on that purpose.
  Treasure Hunter, an ability given to the thief class in Final Fantasy XI Online shows what happens when a mechanic is unclear and when it does not deliver on a purpose.  Based on the name alone, players assumed that this ability increased drop rate of items.  It did not do that as far as anyone can tell and I do not believe that Square Enix ever gave the players a clear answer on what it is sposed to do.    
• A successful mechanic is simple to use and easy to understand.
  The Dragon Punch (Shoryuken!) from Ryu (or any of the shoto characters) in Street Fighter is a good example of this.  It is simple to use and the player clearly understand what it is used for.  At the same time, its easy enough for the opponent to understand what its intent is and react accordingly.
• A successful mechanic does not compromise the aspects other mechanics at work or the gameplay.
  
• A successful mechanic creates engagement from the players.
  Something like Focus Attacks from Street Fighter 4 is very successful at doing this.  Using it in the correct situation makes the player feel smart because it can be used as both a defensive tool or an offensive tool.  At the same time, it can be punished properly by the opponent.  

For example: Bayonetta, Super Street Fighter 4, and Team Fortress 2 all pretty much showcase these mechanic design ideas.  As I have stated previously, these are not hard rules.  But if you look at games that I have mentioned, I think you will find similar philosophies in the design of their mechanics.  

Summon a holy lance that strikes all enemies in your path  for X weapon damage.

Runestones:
Shockwave: (Alabaster Runestone)
Creates a shockwave that inflicts X weapon damage to all enemies around the Knight.

Impalement: (Crimson Runestone)
Impales all enemies with directly in front of you with Divine Lance dealing X weapon damage and slow the movement of enemies by X%.

X – Name Needed: (Golden Runestone)
Reduce the cost of Divine Lance to X Zeal per attack.

X – Name Needed: (Indigo Runestone)
Summon X lances that strikes all enemies in your path for X weapon damage.

X – Name Needed: (Obsidian Runestone)
Each lance leaves behind a trail of lightning that deals X% lightning damage to enemies in their path.

[Another quintessential Knight weapon. A lance is not only cool, it conjures all sorts of different images of jousting events and it kind of defines the nobility of the Knight.]

Summon forth a divine shield to slam enemies directly in front of you for X% weapon damage with a chance to knock the enemy back.

Runestones:
Shockwave: (Alabaster Runestone)
Creates a shockwave that inflicts X% weapon damage to all enemies around the Knight.

Shield Spike: (Crimson Runestone)
Punctures all enemies with directly in front of you with Shield Slam dealing X% weapon damage. Removes the chance to knock enemies back.

Vindication: (Golden Runestone)
Generates X addition Zeal per attack.

Wrath: (Indigo Runestone)
Each attack grants a X% armor increase for X seconds. This effect stacks up to X times.

Repentance: (Obsidian Runestone)
Instead of knocking enemies back, each hit has a X% chance to stune the target for X seconds.

[When thinking about a Knight, a shield bash kind of attack seems quintessential to the class. At the same time, I don't want to restrict the players into using only "weapon + shield" in order to use this attack. So very much like the Barbarian's ability to summon the Hammer of the Ancients, the Knight is able to summon a shield of divine force. This allows the Knight to perform a Shield Slam while still being able to use all of the weapons available to him throughout the world of Sanctuary.]