Sometimes the design articles are good...
http://www.wizards.com/magic/Magazine/Article.aspx?x=mtg/daily/mm/303
Also, unrelated to the above, I saw an interesting comment which made me think about threats and answers in cube.
Generally, in the first couple of turns of the game, the threats are small creatures. This means, early answers are simply things that answer small creatures. Low cost removal, like
Shock or
Doom Blade, or creatures of your own. This is why it is pretty much universally accepted that cubes need some reasonable density of cheap spot removal and early creatures. These combined make the early game very dynamic.
The issue comes when the early game threats are turned on their heads, and completely different answers are required. For instance, a turn 2/3
Tinker'ed giant death robot, or a turn 2 Planeswalker off of fast-mana. These early threats are completely different to small creatures, and demand completely different answers. Now I am sure it is possible to design an environment where answers to these types of threats are as common as answers to early aggressive creatures (I think the key is
Force of Will...), but it certainly isn't the norm, and trying to create a limited environment where early threats are both small creatures and giant death robots is going to be a real challenge.
Legacy is difficult enough to build and design for without being hamstrung by having to draft answers...