The proposal for Cat 5 required bi-directional signals on each pair, with multilevel coding to compress the data. The transceiver needed digital signal processing at very high speeds to make it work. Cat 6 here we come. They have developed test suites for many network types and just reading the descriptions are an education in the problems of interoperability of each network type. It's a really sticky problem, one that even a test standard may not reveal real world problems.
Return loss is the reflection caused by an impedance mismatch.
It's what causes a TDR to give you big reflections from an open or short at the end of a cable. Only if the cable is properly terminated will it suppress reflections. On Ethernet or Fast Ethernet, transmissions are only going in one direction on a pair. If you are transmitting and receiving, the reflections from the far end can be as disrupting as crosstalk from transmission on adjacent pairs. But when you test return loss, you must plug testers into the link.