Thanks everyone for the help, especially nightman for the detailed answer.
The missed incarnation is definitely a huge mistake, I remember holding on to it to wait for after the slam, but I never realized it was that long.
Regarding the dots, we had a couple add pops, I see I did 400k which each dot on burning embers, it likely accounts for a couple casts.
Trinket-wise, on that fight, I had the Metronome (raid finder :/) and a heroic plaguehive, as I tend to switch around a bit
Last but not least, yeah, our overall dps is definitely on the low side, but as I said, we're a very casual guild of friends where not everyone plays optimally, and we're fine with that. That's also why we don't invest too much on the more expensive pots.
So, all in all, beyond the forgotten cape enchant, the main mistake was a missed incarn (ouch). Again, thanks everyone.