ETX is one metric specification. It predicts for each link the expected transmission count per packet. Then it picks the route that has the least combined ETX count. It approximates the ETX by sending a periodic probe (every 1 sec). By dividing the actual received probes by the number sent, you get the delivery ratio. The inverse of it is ETX. It's very simple to understand and easy to implement. It adds very little to the routing states.
ETX has its own limitations. It only interacts and reacts to mobility as good as it updates the routing states, which is delayed by WST (weighed settling time). The main essense of using transmission count is a step further than hop count; however, it doesn't go into the size of packets. It would more ideal to have something like ETX/bit.
The paper presents a simple understandable routing algorithm and implementation. It makes a argument and brings attention to the problem of min hop count. Although it would be nice to see a more comprehensive paper on routing metrics, this paper is a pretty good introduction.