espressme wrote:What makes an E-61?
The term "E61 group" is frequently misused. Some attribute the term to the saucer shaped grouphead, some attribute it to the thermosyphon design, others attribute it to the automated preinfusion. While the first two attributions are common embodiments of the implementation, only the latter attribute is part of the patent claim; see E61 Group Espresso Machine: Is its reputation justified? and Is there a purpose for the E61 middle brew lever position? for more details.
The short answer to "what makes an E61?" is "the alternately seating valves." The familiar grouphead shape and its thermodynamics are just the particulars of an example embodiment of the patent.