Things that were completely needless to the plot or could have easily been changed to not recycle fanboy material, but were almost certainly left in to garner fanboy love:
Blue Milk A piece of set dressing that is just sitting there?
Yavin 4 Rebel Base with long lingering camera angles of the guys scanning incoming ships in towers This movie takes place hours before a New Hope. You're going to see the same locations.
Grand Moff Tarkin - distracting plastic doll version My grandfather wife and father in law all saw the film and had no idea he was CGI.
"Just watch yourself" guy and walrus/butt face Agreed
General Dodonna If he's at the base in 4 he's going to be there 3 hours earlier.
Mon Mothma Same ^
Bail Organa It makes sense that he would be there if the alliance is in contact with Leia, plus he adds context to how the Rebellion knew and contacted Obi Wan.
Leia Organa - distracting plastic doll version Same as Tarkin. Plus she's the one that passes the plans to R2 so it makes sense that she would be the last one we see with the plans.
C-3P0 Meh. It makes sense that they would be at the Yavin Base. I can see the complaint that they weren't involved in the story of Rogue One, but did they really detract from the film?
R2-D2 Same ^
"I've got a bad feeling about this." Agreed
"It's a trap" Agreed
AT-ATs Why? If they are part of the Empire's arsenal why wouldn't they have a few on Scarif?
T-15 reference (hurr durr so clever) No casual viewer would even notice that one.
Holochess It's a game in their universe why is it odd people would be playing it? Should they have been playing a different fictional game?
Rouge Squadron Red and Gold leaders - distracting CGI plastic doll versions Those weren't CGI that was archived footage from A New Hope they found when they visited Lucasfilm Ranch. Complaining to complain?
References to Antilles Edit: Didn't realise that they were referencing a different Antilles. Shame on me.
Plus so many more.
I would also put Vader down as a needless callback, because he has no effect on the story at all. His speaking scene was pointless and should have been left on the cutting room floor. I would also say that his corridor scene was a great example of whoring out. Would it have been classier and likely more effectively menacing to show Vader's ships coming in to clean up the rebel rabble without actually showing Vader? Yeah probably. Would that have caused fanboys to go "OMG sooo cool guise"? Probably not.
*EDIT* I would be remiss if I didn't commend Mads Mikkelsen on his performance. He stood out as a particularly convincing and at times seemed like the only person who was aware this wasn't a spoof of Star Wars but was, in fact, a real movie.