Pullup resistors are required on the I2C lines SDA and SCL. The typical resistor is 4.7K (4700 Ohms). Many modules come with the pullups installed on the module board. One or more of your boards probably has the pullups installed. Also, the Wire library enables the internal pullups on the SDA and SCL pins, although on some boards (ie, Uno) the internal pullups are pretty weak and may not be enough by themselves. You can have too much pullup if many modules have resistors installed, the parallel resistance can be too low.
From the Sparkfun I2C tutorial:
Unlike UART or SPI connections, the I2C bus drivers are “open drain”, meaning that they can pull the corresponding signal line low, but cannot drive it high. Thus, there can be no bus contention where one device is trying to drive the line high while another tries to pull it low, eliminating the potential for damage to the drivers or excessive power dissipation in the system. Each signal line has a pull-up resistor on it, to restore the signal to high when no device is asserting it low.