Generally speaking, there are two things I take into account when making a change my cube : Holism, and Power.
Power is defined by the raw power of the card. Time Walk is incredibly powerful, Woodfall Primus is very powerful. The power of a card is it’s potency to do something insane, or to break a rule of magic. Obviously these are very rough guides of what power is, but you get the idea. To me, power is always a positive value, from 0 , like bog hoodlums, to 10 , ancestral recall.
Holism is a greek word, meaning that the cube is different from the sum of all it’s cards. In layman’s terms, it’s how well your cube functions as a set. This means how well certain cards come together to form archetypes, whether you’ve got enough artifact hate, whether you’ve got a varied metagame. There are varying levels of holism in a card as well. For example, Darksteel Colossus is negatively holistic, since almost no deck will ever be able to run it.
Flickerwisp is a very interesting card in this way. You have a fairly high powered card, an evasive 3 power for 3, which can temporarily remove a blocker. However,you can also use it to blink your own creatures, to abuse their ETB/LTB effects.
For most cards, you have to find a balance. Tinker is a very powerful card, and can have an archetype built around it. The trouble comes when your deck is bad when you don’t find tinker, and I think that’s the kind of ‘build-around’ card which should be lost in cube. Build-around cards can define your cube in a rotisserie draft, where they can be abused to their fullest extent. Tinker is a good example of a build around card, where you’ll try and draft some massive artifact creature to cheat into play early ; However, if you don’t run many robots in your cube, Tinker will lose value. Upheaval and Wildfire encourage you to draft low casting mana artifacts, but in a powered cube, these are likely high picks anyway. The ease with which you can build around a card, given it early, is a factor in it’s place in cube.
An example of a powerful ‘build-around’ card which shouldn’t be in cubes is Oath of Druids. OaD shouldn’t be run in cubes because it encourages you to run very few creatures, which is pretty hard in green, and then relies on you drawing OaD. Without OaD, you’re probably left with a GX control deck, which isn’t necessarily bad ; UG control gets played fairly regularly here, but rarely any other type of GX control arises, so you can assume your GX control deck won’t be very good.
As you can see, for the most part, it’s pretty easy to see the balance between how a card interacts with the rest of your set, and how powerful a card is when it doesn’t come together. Then there are some harder examples. Tezzeret, the Seeker, for example, should be incredibly powerful the smaller your cube gets ; he can tutor up moxen, sol ring, and can untap them at will, functioning as a garruk, but with better ramp. On the flip side however, the value of tutoring these cards decreases as you go down in size, since there’s a higher density of them anyway, and the relative power level of your blue section increases faster than the power of tezzeret targets. In the same way, Ranger of Eos should get a lot better as your cube gets smaller, since there’s a higher density of powerful one drops, like Kird Ape, or Isamaru. The trouble is, as you get smaller, the number of cards at any given mana cost decreases too, so when you reach 360, where RoE should be at it’s best, it’s not as good as the other heavyweights at 4 mana in white ; Calciderm ,Hero of Bladehold and Kor Sanctifiers. I’d actually say RoE isn’t too far behind though, so hopefully it’s time will come.
Some entire cubes choose to be more holistic than powerful. Tribal cubes, artifact cubes, and other theme cubes are testament to this. Your cube is a set of your choosing ; that’s basically the definition. You need to find a balance between how powerful the cards you add are, and how well those cards interact as a set ; Where that balance is will define your cube, and how enjoyable it is for your group.
Thanks for reading,