Starting your talk (or lecture!) with a bunch of definitions fails to take into account the previous point, about starting a talk by motivating it, and it does so in the worst possible way: you present your listener with something that he cannot use at the moment, and expect him to put some effort in order to understand it. The implicit promise that this definition will become useful to him later is rather not enough to motivate this effort, especially as experience (at least mine) suggests that this is almost surely going to be a bad talk.
Putting definitions at the beginning has further drawbacks: it is easier for the listener to understand and remember a definition if they know what they are going to use it for.
A. Georgakopoulos: How to give a talk that is not too bad.