The address can be found out with a utility code.
Can be found is correct, but usually you look that up in the datasheet.
I2C has the additional advantage that by design you're able to connect devices with different voltage levels (5V and 3V3) to the same bus without destroying them because the lines are pulled up by resistors and not actively by the master or slave devices.
COM port style TX/RX line pins that run at 5V while PC com port runs at 12V.
PC (better RS232) has +/- 3-12V, the negative levels are usually the bigger problem than the voltage but you should never connect TTL UART devices directly to RS232, that's correct. This interface is asynchronous which means that the timing is more critical. UART allows two way communication (bidirectional) with just two wires (they're usually counted without the power wires Vcc and GND). Using chips as the MAX485 you can use these UART ports to communicate with RS-485 devices (used in ModBux for example).
I2C has two defined speeds (100kHz and 400kHz), SPI usually is much faster with speeds in the MHz range while the UART interface usually is the slowest variant with common speeds in the 1-100kHz range (although i can go higher up to some MHz).