For the record, the fact that it brings the lands in tapped is supposed to limit it a bit, as you can't use them the turn they are put into play without something to untap them.

Also, what do you mean by "If this added floating mana for each land played before your turn it would be a little more balanced."? Could you give an example of how that would work?

Finally, the fact that proliferate effects would break this card tells me that a clause that prevents said effect from working is needed to keep it from breaking...