Ethernet: Why is the link status "down" when there is a speed mismatch?
Different signalling rates are generally completely incompatible with each other.  Both ends of the link have to be running at the same speed for the link to come up.  Now, it's possible for one end to support faster speeds than the other end so long as they are either explicitly configured or they autonegotiate to a common rate.  For example, if you connect a computer with a 100M ethernet card to a router with 1G ports, the link should autonegotiate to 100M and come up.  

For straight serial signalling, the receiver has to lock on to the data stream.  The way the receiver is built, this is only possible if the actual data rate is within a few percent of the expected data rate.  Raw serial rate for 1G ethernet over fiber is 1.25 Gbps.  Raw serial rate for 10G ethernet over fiber is 10.3125 Gbps.  Generally 10G PHYs can be configured to run at 1.25 Gbps, but it is not possible for a deserializer configured for 10.3125 Gbps to lock onto a data stream running at 1.25 Gbps.  

High speed serial links also use line codes to provide certain electrical characteristics, such as DC balance and guaranteed minimum transition density to aid in clock recovery.  Different rates can use different line codes.  1G ethernet over fiber uses the 8b/10b line code.  10G ethernet over fiber uses the 64b/66b line code.  You can't receive 8b/10b line code with a receiver configured for 64b/66b, even if the serial data rate is the same.  

Ethernet over copper cabling has the same set of issues, but there can be very substantial electrical differences as well.  10 Mbit ethernet transmits on one pair with Manchester encoding to transmit the combined data and clock signal.  100 Mbit ethernet transmits on one pair with a 4b/5b line code and MLT-3 (multi-level transmit), transmitting with 3 different voltage levels.  1 Gbit ethernet uses some fancy echo cancellation signal processing to transmit on all four pairs in both directions at the same time with PAM-5 (pulse amplitude modulation), transmitting with 5 different voltage levels.  

So as you can see, if the settings do not agree, the receiver is going to be horribly confused and no data will be transferred.