On colorless decks

Colorless decks have three big issues:

• Most cards have a big upfront investment cost and are expensive for their effect.
• There are very few cards available that can efficiently interact with what the other players are doing.
• Many decks pack one or more mass artifact removal spells. We have little to no protection against these.

It's important to select cards for a colorless deck that can generate value the moment they are played, this is even more important if their initial investment cost is high (e.g. Citanul Flute, Myr Turbine, ...).
Cards like Darksteel Forge, Mirrorworks, Loreseeker's Stone ask for a big upfront mana investment and do nothing or very little the turn they're played.

There are in practice only 2 good (but expensive) mass removal spells in our "color": Ugin, the Spirit Dragon and All is Dust.
Because this is an inherent weaknesses of the deck archetype, it's not unreasonable to focus on our strengths and lean more on the other players for removal.
Rather then packing inefficient single target removal, we can use some of these card slots to improve the other aspects of the deck.

The only defense against mass artifact removal is to not over-commit to the board and to keep our hand full.
It's important to include enough sources of direct and indirect card advantage.
Even then, cards like Null Rod, Stony Silence, Collector Ouphe or even Titania's Song can cripple the deck to the point where winning becomes almost impossible.

Colorless decks have one very big advantage:

• It's impossible to get color screwed.

We can play all the efficient colorless mana ramp and as many utility lands as we want.
This means we can often keep pace with the green big mana decks.

Combo

Combinations are the most effective way to win with a Karn deck.

Combo cards were selected for their usefulness outside of the combo and their mana cost.
Cards like Nim Deathmantle, Krark-Clan Ironworks, Rings of Brighthearth, Staff of Domination and Salvaging Station can at least provide some value outside of their role in a combo.

Most combo turns end with us playing Walking Ballista for an arbitrary amount of mana.
Other potential combo finishers are Mimic Vat and Myr Turbine. (Both with Voltaic Construct.)

Infinite combo's:
Karn, Silver Golem + Voltaic Construct + Basalt Monolith/Everflowing Chalice/Gilded Lotus/Grim Monolith/Mana Vault/Thran Dynamo
Karn, Silver Golem + Salvaging Station + Mana Crypt
Nim Deathmantle + Krark-Clan Ironworks + Myr Battlesphere
Rings of Brighthearth + Basalt Monolith
Karn, Silver Golem + Salvaging Station + Sol Ring/Mana Vault + Skullclamp
Metalworker + Staff of Domination
Sensei's Divining Top + Cloud Key/Ugin, the Ineffable + Mystic Forge

And many more variants.

On using Wastes

Basic lands are often undervalued.

They allow us to use cards like Burnished Hart, Solemn Simulacrum, Wayfarer's Bauble, Thaumatic Compass, Extraplanar Lens, Thawing Glaciers and even Endless Atlas.
These cards provide card advantage and allow us to ramp in ways that are usually not available to artifact decks.

The only non-basic lands that make the cut are sources of card advantage, land destruction or lands that are very powerful like Ancient Tomb or Glacial Chasm.