Gotta be the valve body system. One word (although this kinda applies to the whole gun): Modular!
Next to that trigger frame, then barrel system, then pump insert system, then stock class feed tube/cap system (so simple but so nice, even compared to Carters and especially CCI and LAPCO).
So many little things that go into the garg to make is a wonderful gun to shoot, maintain, setup and upgrade! TBH the breech/feed tube integrated system was never a selling point for me, although the ease of cleaning the guts does make it, well, awesome.
But yea, favorite aspect is the valve system.