First an annoucement:
Yes, the first card removed from the cube because it was too good. I'm aware that goes against the principle of power cubes, but there's a level at which cards win games by themselves too frequently and there are not enough answers for them. Letting them go makes for a better, more fun and less frustrating environment.
As we all saw over a few drafts, good planeswalkers are overpowered. Guys like Chandra Ablaze and Tezzeret the Seeker (two of the worst planeswalkers printed in my opinion) are OK. They do change some games, but other spells at their costs often do so. They are situational though, and one has to put some thought on whether to run them, build around them, etc.
That is not the case of powerhouses like Elspeth, Knight-Errant, Jace, the Mind Sculptor or Garruk Wildspeaker. They share some characteristics that makes them bad cards to have in playing environments. In fact, I believe Magic designers did not execute them well, and development wasn't competent enough to spot these problems - which are circumventable.
1. Planeswalkers are too versatile
Cards in Magic have roles. Walls block, finishers end the game, counterspells counter spells, removal removes permanents. Planeswalkers often do three of these. Unlike charms, they do not do one of these, depending on the game. Instead, they may do ALL in the same game. Jace, the Mind Sculptor draws cards and digs into the deck with his first ability, bounces creatures for defense with the second and finishes the opponent with his ultimate. Now find an enchantment that lets you do all these things.
2. There are not enough answers to planeswalkers
First, there are no cards that say "Destroy target planeswalker." They can only be directly taken out with generic permanent destruction, which is remarkably rare and usually expensive. Oblivion Ring is the most obvious answer, and has been used extensively as such. The only other option that does not require an obscene amount of mana is Vindicate, which can only be played in a handful of decks.
3. Planeswalkers are defensive by nature
The other way to kill planeswalkers is by dealing damage to them. What do decks need to do to deal damage? Attack! This need to attack forces the opposing deck into an offensive position. If your defenses overcome the opposing offense, then the opponent is at a lose-lose situation - either they attack and lose the clash or do not attack and let you build up your resources with your planeswalker. That's why control decks and planeswalkers are intrinsecally sinergistic. Even if you are playing an aggressive deck, if you play a planeswalker it will frequently be worth leaving your creatures to block and defend it, because having one online is so powerful.
4. Planeswalkers have been aggressively costed
Darksteel Colossus is extremely powerful, usually winning the game when it's not dealt with. The same applies to Akroma, Angel of Wrath, Blazing Archon, Decree of Silence, Eternal Dominion and several other cards that were too far into the alphabet for me to get to. Why can't they be called broken though? Because their mana cost is so high that they can only be cast in late stages of the game, the opponent has plenty of time to prepare and have a shot at killing one first. Now, if we consider that an environment when aggro decks reliably kill before turn four is too fast, wouldn't an environment that requires aggro to kill before turn four be too harsh to them? All planeswalkers could have been printed as they were. Only costing at least five mana. This may sound like a small tweak, but to aggro decks, it's the equivalent of a Time Walk every game.