It's actually not even the ability to shapeshift into anything out of any of the sourcebooks that breaks the game. That's powerful, sure, but alone, as long as the monsters are fairly reasonably rated for challenge rating, and if the duration isn't all day, it's not a big deal. The problem is the ability to stack parts of the monster's stats with your own existing christmas tree of bonuses - Natural AC stacking with everything, including your enhancement bonus to natural AC, Armor bonus, Deflection bonus, Insight bonus, Unnamed bonuses, etc. If you do enough research, you can create a monster.
Theoretically, the loss of the christmas tree of bonuses in 4E should account for breaking a lot of that down. Make it a daily power with an encounter long duration, and there you go. Some good wording of the spell and typing of abilities should allow any totally game-breaking abilities to be excluded. From that point, as long as they've done a reasonable job of rating the challenge of the monsters, nothing too game-breaking should ensue. The GM is always free to ban forms at his table. As to the time factor, the rule could simply be the way it is at my table: if you don't have a stat block with your bonuses properly factored in with the new form handy you can't change into it.
But all of that goes against the idea of the aesthetic of simplification and requiring little prep time for 4E. Furthermore, the fact that WotC believes that additional sourcebooks provide a huge power boost to the shapeshifter speaks volumes of their lack of desire to change their production process so that splatbooks are vetted better and don't contain hugely unbalanced options, or so that errata which fixes major problems comes out in a timely manner. Which might make sense from a business perspective - the current system is profitable for them. Not only that, but well-worded spells and typed abilities are just plain hard to do. When that many people are banging on your system, it's tough to think of everything ahead of time. Summoning is in the same boat.
So, ultimately, the choice to nerf shapeshifting and summoning is somewhere on a continuum On one end, this is a great decision based on sound business principles and self-knowledge of what they can and can't do properly given their deadlines. On the other end, it's designer laziness, taking the easy way out by just not attempting the parts that are hard.
Personally, I fall towards the designer laziness end of the spectrum. I have no doubt that they think the problem is insurmountable, but any problem is insurmountable when you refuse to make the changes required to fix it. It's really really tough to lose weight, for example, if you refuse to change your eating habits. Not only that, but my feeling is that I don't pay a company for RPG rules for them to tell me that something is too hard to make work so they didn't try. I pay them for rules that work and do the things I want them to.
But I'll wait for the finished product to pass final judgment. And anyway, maybe on later supplements they'll get it working.