Looking for more specifics as to what might define a "deep threat", I took the TOP 40 pass-catchers for 2011 in terms of yards/reception (minimum 50 targets), then computed their "Air Yards"/reception by subtracting their YAC from their total receiving yards - and then dividing THAT by their receptions (and sorted by "air yards"). I also calculated the % of their catches that went for 20+ yds and their overall catch rate (CR). For comparison, I included Ochocinco and Tate (from 2010), and Brandon Lloyd (who placed just outside the Top 40 in YPC). Here's a sampling of what shook out:
M. Floyd ----- 43 rec, 856 yds, 19.9 ypc, 37% (20+ yds), 16.4 air yds/rec, 61.4% CR
Riley Cooper - 16 rec, 315 yds, 19.7 ypc, 31%, 16.2 air, 47.1%
Johnny Knox -- 37 rec, 727 yds, 19.6 ypc, 32%, 15.4 air, 53.6%
Ochocinco ---- 15 rec, 276 yds, 18.4 ypc, 20%, 15.0 air, 46.9%
V. Jackson --- 60 rec, 1106 yds, 18.4 ypc, 35%, 14.7 air, 52.2%
Den. Moore --- 33 rec, 618 yds, 18.7 ypc, 36%, 14.5 air, 43.4%
Dem. Thomas -- 32 rec, 551 yds, 17.2 ypc, 28%, 13.7 air, 45.7%
B. Tate (2010) 24 rec, 432 yds, 18.0 ypc, 33%, 13.3 air, 52.2%
B. Hartline -- 35 rec, 549 yds, 15.7 ypc, 23%, 13.1 air, 52.2%
R. Meacham --- 40 rec, 620 yds, 15.5 ypc, 23%, 12.9 air, 65.6% (!)
DeS. Jackson - 58 rec, 961 yds, 16.6 ypc, 26%, 12.3 air, 55.8%
Jordy Nelson - 68 rec, 1263 yds, 18.6 ypc, 28%, 12.3 air, 70.8% (!!)
Torrey Smith - 50 rec, 841 yds, 16.8 ypc, 28%, 12.3 air, 52.6%
C. Johnson --- 96 rec, 1681 yds, 17.5 ypc, 33%, 12.1 air, 60.8%
AJ Green ----- 65 rec, 1057 yds, 16.3 ypc, 29%, 11.8 air, 56.0%
Brandon Lloyd 70 rec, 966 yds, 13.8 ypc, 21%, 11.7 air, 46.7%
S. Smith (CAR) 79 rec, 1394 yds, 17.6 ypc, 37%, 11.6 air, 61.2%
Jacoby Jones - 31 rec, 512 yds, 16.5 ypc, 29%, 11.6 air, 48.4%
Victor Cruz -- 82 rec, 1536 yds, 18.7 ypc, 30%, 11.5 air, 62.6%
L. Fitzgerald 80 rec, 1411 yds, 17.6 ypc, 31%, 11.5 air, 52.3%
A. Boldin ---- 57 rec, 887 yds, 15.6 ypc, 26%, 11.3 air, 54.3%
A. Brown (PIT) 69 rec, 1108 yds, 16.1 ypc, 26%, 11.1 air, 56.1%
L. Robinson -- 54 rec, 858 yds, 15.9 ypc, 22%, 11.0 air, 66.7%
B. Marshall -- 81 rec, 1214 yds, 15.0 ypc, 25%, 11.0 air, 55.9%
M. Colston --- 80 rec, 1143 yds, 14.3 ypc, 19%, 11.0 air, 74.8% (!!!)
Hakeem Nicks - 76 rec, 1192 yds, 15.7 ypc, 22%, 10.9 air, 57.1%
M. Wallace --- 72 rec, 1193 yds, 16.6 ypc, 25%, 9.7 air, 63.7%
R. Gronkowski 90 rec, 1327 yds, 14.7 ypc, 24%, 7.5 air, 72.6%
A couple of observations about these numbers.
Many of the guys who are widely considered "deep threats" aren't really, on average, all that far down the field when they catch the pass and their YPC is supported in large part by what they did after the catch.
Aside from a couple notable exceptions, the catch rates for these "deep" guys are pretty low, relative to the 70%+ posted by the Pats top three pass-catchers (who were all among the Top NINE in YAC). Deeper passes are simply more difficult to complete (even shorter sideline routes are lower percentage than most routes over the middle).
Ochocinco's receptions were much further downfield than any other Pats pass-catcher (and even further than Tate's); IOW, Ocho WAS the Pats "deep threat" in 2011. Tate really wasn't all that shabby as a "deep threat"; his problem was that he couldn't run any other route consistently well, which minimized his overall effectiveness (when he was on the field, the defense pretty much knew where he was going). In contrast, Ocho DID run some short/intermediate sideline routes (on which he DID get open), but Brady didn't throw to him very often. Again, why would he when he had a much higher percentage pass available to a guy over the middle who was far more likely to get big YAC?
I don't think the reality of a "deep threat" is what most people imagine it to be or that any new guy is going to have the dramatic effect on the Pats' passing attack that most folks appear to think he will - unless they can somehow come up with a Malcolm Floyd, Robert Meacham or Jordy Nelson (or maybe bring back Tate). Even then, there's no guarantee that Brady would actually USE such a guy more often than he did Tate or Ocho.