Lots of interesting arguments.
Not sure if this has been mentioned before, I think another thing that can make a game feel overly-short is length of time waiting for it to release or expecting developers to make a sequel longer than its predecessor (in certain cases).
I'll refer to Modern Warfare. People complained that the campaign was short (it really is), but I mean, it was the first of the MW series and the game ended on a cliffhanger, so people lived with it. People expected the sequel to be longer and with the time spent developing it, the only thing people speculate was, "Gahhh, its been so long, they MUST be making it much longer, and MUST have heard our desires!" Aaaand, they didn't. Modern Warfare 2 was about 4 missions longer, I believe, and it ended on yet another huge cliffhanger.
Which brings up another point: paying 65 dollars for online multiplayer. The campaigns in some recent shooters are just an excuse to call it a "full" game. I want my money to go equally to each aspect of the game or at least be an unbalanced split that is fair. When I bought MW2 for $65 (I think it was raised 5 dollars to $70 not including tax because of how hot the game was going to be on release)... turns out I paid about 55 dollars of that on multiplayer because the campaign was just a joke. The lead up from "crap might be going down soon dude," to "CRAP ITS GOING DOWN DUDE," to "crap, let's throw them another cliffhanger dude" happened in the shortest amount of time possible.
Was it fun for the most part? Yeah, sure, but, right as the going gets good and you start finding out the big plot points in the game, well, it ends. The campaign is barely 4 hours long, if that.
By the way, I ended up selling MW2 because of its weak multiplayer. I'm sure some of you have heard of its lack of dedicated servers (host migrations and lag almost every match, and half the time, you can't join a lobby), and its series of glitches and exploits. Aaand, that is why I bought Bad Company 2.