As far as artifacts and lands to avoid, there's not much. I mean, as long as you guys have fun with the cards and the cube is an enjoyable experience, you're doing it right. I would caution against overdoing lands that enter the battlefield tapped though, if you want aggressive decks to be playable in more than one color.
Cheers, and happy cubing!

While there are less combinations of effects that add up to card advantage, I think the Petal effect and the Study effect are valuable in more situations, and often have higher ceilings in the situations where they shine.
I like Wall of Blossoms quite a bit, but Wall of Roots is an outstanding cube card.
Krasis > Nissa
Carver > Scavenger > Engineer
Elder > Vandal
Epiphany > Expertise
Nissa >> Koma
Mammoth ≥ Gargaroth
Entity > Greatshark
Horizon Lands > Pathway Lands
Pathway Lands > Cycling Lands
Staggershock > Blast
In unpowered, I'd go with Revelers. In powered, I prefer Grudge.
@Sky - Ya, I can see that. Like I said, I don't think they're bad and I don't fault anyone for playing Doom Blade variants. They're just not as appealing to me as they used to be, and we've opted for different forms of removal that we've found to be more effective.
They're in no way bad, but they started to feel less and less efficient over time, so I prefer the other options. And black has so much removal that I never feel short. But there's always a chance that cards like GFTT and HA could come back in at some point. They're perfectly fine cards.