Hi Carl!
There were several elements to this that caught my ear - other than the instrument choices - which worked really well together. The first was the syncopated feel that the guitar and piano were creating. The second was the use of the dim (or was it m7b5?) at roughly 0:49 and 2:19 with the escalating feel. I think I heard a X major to x minor chord change in there as well. The third is the combining of long-held-noted cello over the guitar and piano playing with 1/8th or 1/16th notes. I first learned of this several years ago - that it pays for good mix and balance that not all instruments are 'busy' at the same time.
A beautiful piece!
Andrew