You are talking about clips of him in space. He is great in space. He doesn't have quick feet out of his breaks. Show me great routes in tight space. That is what he would need to do to be a threat for 100 catches a season.
In my opinion, he is much more like TO than Fitz and Marshall. A poor man's TO, and I don't say that to be insulting.
Not really interested in dragging this along much further, as I've said most of what I want to say on the topic of Thomas, but I don't think your take has much basis in reality. Thomas is not some slow-footed lumbering goon who can only run a go route. Here he is coming off the line of scrimmage and getting good separation on a 1 yard out route.
https://www.youtube.com/watch?v=BoKy-mhxe10
I'm not seeing any stiffness or lack of foot quickness there. Here are some other good clips that showcase his running ability:
http://www.nfl.com/videos/nfl-fantasy/0ap2000000067999/QB-Manning-to-WR-Thomas-25-yd-pass
http://www.nfl.com/videos/nfl-fantasy/0ap2000000067956/Raiders-defense-fumble-recovery
Teams don't usually throw the 1 yard bubble screen to players who can't run in tight spaces. You use this play to get your best playmaker the ball with a chance to make something happen. Thomas has delivered some massive gains on these screens this year (including a pretty nice 70 yard score against Pittsburgh).
And this clip really shows how fast he is off the LOS. He eats the corner for breakfast.
http://www.nfl.com/videos/nfl-game-highlights/0ap1000000078036/Thomas-38-yard-catch
Here he is running a pretty challenging stop-and-go route in which he gets the corner to bite:
http://www.nfl.com/videos/nfl-game-highlights/0ap2000000077918/Thomas-30-yard-one-handed-catch
You can say that Manning makes him, but his talent just pops off the screen. It's hard to look at what this guy does on the field and not think he's one of the very best in the league. He passes the eyeball test. The numbers are there. The physical metrics are there. He was a first round pick. There's really nothing missing in his game.
As for the TO comparison, his single season career high in yardage is 1451. Thomas is on pace for 1552 this season. Granted, TO missed two games that year, but Thomas is basically producing on par with TO's best year and I'd guess that he's doing it on far fewer targets (although I can't find the target info from that far back).
Having Peyton chucking passes his way helps in the sense that his opportunities are good when he gets them, but the flipside is that he doesn't get very many chances compared to other top NFL receivers. Here are the top 10 NFL leaders in receiving yards along with their total targets to date:
Reggie Wayne - 757 yards - 92 targets
Wes Welker - 736 yards - 83 targets
Demaryius Thomas - 679 yards -
57 targets
Brandon Marshall - 675 yards - 80 targets
Percy Harvin - 667 yards - 79 targets
Victor Cruz - 650 yards - 89 targets
Calvin Johnson - 638 yards - 74 targets
AJ Green - 636 yards - 73 targets
Vincent Jackson - 626 yards - 66 targets
Roddy White - 591 yards - 57 targets
By and large, yardage is a function of opportunities. Thomas has more yards than Brandon Marshall, Percy Harvin, Victor Cruz, AJ Green, and Calvin Johnson, on approximately 65-75% of the targets. So I think he could quite easily go to a bad team, get a larger slice of the pie, suffer a reduction in target quality, and still put up top 5 FF stats.
For me, Thomas is a top 10 pick in PPR dynasty startups. I think that will become the consensus as the season rolls on and he continues filling the box scores and highlight reels every week.