As SuperNomario and Number45Forever said, he has great hands, he can be your PR, plus he is a pretty smart player as well as an above-average (for a WR) downfield blocker. I think teams are seeing the success of a good slot guy, like Edelman or Welker before him, and will pay for that type of guy. I also think that teams with a good running/scrambling QB would find DA's downfield blocking to be particularly useful.
That said, the closest recent comp to a FA signing is probably Mike Wallace who signed for 11.5M/2 yrs. Wallace of 5-6 years ago was a higher top-end guy than DA ever was, but Wallace of late isn't much different than DA.
2015 Stats:
DA: 65 rec, 87 tgts, 648 yds, (10.0 avg), 3 TDs
MW: 39 rec, 72 tgts, 473 yds, (12.1 avg), 2 TDs
Wallace is a bit bigger (but only 1" taller as listed) and one year younger.