It's definitely the ability to parse expressions like "<file>:<lineno>" and open files at the right position, it essentially makes it integrated with many many unix tools without actually having to write any integration code.
The ease of piping the selection through an external program is a close second.
However I do not use it.
What's funny about acme is that it's a perfect metaphor for the entire plan9: it shows you how striving for a simple, beautiful design can create new, great solutions to old problems.
But at the same time it will also kill the adoption of the product completely: acme doesn't support any keybindings (doesn't fit the desing), no syntax highlighting (not the editor's job to parse code), nothing but bitmapped fonts (implementing externally defined standards is a chore), tags file support (doesn't fit the design), incremental search (doesn't fit the design), horizontal scrolling (makes it complicated to use the same editor control for the tags too), configuring the color scheme (no), weird non-standard behaviour of up and down keys (we just like it better this way)...
Bringing any of this up to a 9fan will also be a great metaphor for the friendliness of the community.