Believe it or not, fellas, I think Door of Nothingness is the best kill for my Cog deck. First of all, an updated decklist:
CogDoor v3.1
Creatures (14)
4 Trinket Mage
4 Auriok Salvagers
4 Etched Oracle
2 Bringer of the Black Dawn
Cogs and Mana (24)
4 Chromatic Sphere
4 Wayfarer's Bauble
4 Pentad Prism
4 Engineered Explosives
1 Welding Jar
1 Necrogen Spellbomb
1 Aether Spellbomb
1 Pyrite Spellbomb
1 Sunbeam Spellbomb
2 Salvaging Station
1 Door to Nothingness
Land (22)
1 Ancient Den
1 Seat of the Synod
1 Vault of Whispers
1 Great Furnace
1 Tree of Tales
4 Plains
4 Island
3 Swamp
3 Mountain
3 Forest
This deck rides on the power of Auriok Salvagers and Engineered Explosives. After many games, I feel these are the two best cards in the deck, and together they produce an "inevitable board position" where I can simply blow up whatever my opponent topdecks. (There's a surprisingly low number of 4cc permanents to kill). This has happened in numerous playtest sessions and at Friday Night Magic, too.
I originally had 1 Rude Awakening as a Black Bringer tutor kill card, but Door to Nothingness is better. Why? Let me explain.
With this deck, you are constantly building your manabase by sac'ing Baubles and fetching artifact lands with Trinket Mage. You want 5 different colors on turn 4 if possible. As the game progresses, you naturally build up to 2 sources of the 5 colors, with an emphasis on white to use Auriok Salvagers. You want gas for your Salvagers; these guys are stalwart.
This deck wants to control the table with Explosives and various Spellbombs. It gains cards through Salvagers, Etched Oracle, and Salvaging Station. I'm blowing stuff up and drawing cards. While squeaking damage through with Trinket Mages and Auriok Salvagers would be nice, it's a low priority to simply staying in control.
Door to Nothingness fits in here perfectly. I don't really need to kill my opponent by tight playing of damage sources. Door allows me to not worry about lots of cards, like Pulse of the Fields (an absolute nightmare card).
When I drop Door, it has Welding Jar backup. While you may think it gets raped by artifact removal, once I'm in control and Salvaging Station is going nuts, Welding Jar protects most of my table. The only artifact kill spell that can hit is Oxidize. And if you take a look at the decklist, there are plenty of Oxidize targets, so it's unlikely my opponent will hold it for the one turn window Door gives them before the kill.
This deck accels against LD strategies because of the abundance of mana, Salvagers to recur Wayfarer Baubles and Artifact Lands, the ability to remove Arc-Sloggers with Engineered Explosives (and sometimes life-gain with Sunbeam Spellbomb).
This deck is bad against anything that packs Akroma's Vengeance, and most control decks in general that just sit and do nothing but play lands. This deck has no way to punish or clock a slow control deck, and as a result it (usually) plays the control game worse than they can. You usually end up getting Mindslavers and killed with Decree tokens or Darksteel Colossus. This deck is a Mirrodin Block Constructed deck anyway, so don't expect to win against such shenanigans.