Magic items can be fun and powerful without taking a wrecking ball to game balance. In fact, the vast majority of the magic items in Next are very well balanced. There are just a few broken exceptions, the belt of giant strength being one of them. I think it's particularly important that the magic items in the core rules be well balanced, since they will serve as the basis for comparison for all of the magic items that come after. If people want to make game-breaking items for their own games, they can do so at their own peril. But for the core rules of the game, balance should be paramount.
By what standard do I determine that the belt is broken? Comparing it to other items. The very best magic sword can give someone a +3 to hit and damage. The belt of storm giant strength can grant a +4 or more to hit and damage, and this bonus applies no matter what weapon the character is wielding. A character wielding two weapons, for example, normally would have to have two magic weapons in order to get a bonus to hit and damage with both weapons. A belt of giant's strength, on the other hand, applies to any or all weapons a person uses (as long as they benefit from str, obviously). The bonus goes even further than that, since it applies to ALL strength rolls and saving throws.
The effect gets even more powerful for those who had a low strength to begin with. A puny person with an 8 Str can put on one of these belts and suddenly become one of the mightiest warriors in the world. He might even have a better attack bonus than the fighter character who doesn't have the fortune of owning one of these belts! If he had "just" got a magic weapon, on the other hand, he'd be a bit better at fighting, but still at a distinct disadvantage.
You can tell when a magic item is too powerful when it becomes a "must have" item or when it's plainly superior to other items. I can't think of any other item I'd rather have for a fighter-type character than this. Nothing else even comes close.