The most obvious physical change is the big new ATX plug, which now features 24 pins instead of the 20 of the previous version.
Classic ATX plug (left) and the new ATX 2.0 plug
Makes bigger smaller again: 24 to 20-pin adapter
Clever alternative: unpluggable expansion
The four new pins feature +12V, +5V, +3.3V lines and an additional ground. These render the older AUX connector superfluous, so the new standard no longer includes one. The rest of the leads remain unchanged, meaning that the two standards are compatible to a limited extent. You need an adapter to be able to use the 24-pin plug in older motherboards, and most power supply manufacturers include one for that purpose. The reverse combination is also possible, as the 20-pin plugs fit into the big connector too.