Go over to your set of games and open them up and check the manuals, see which ones are in full colour, which have more than five pages, which include gamelore and which include a notes section at the back. I've always felt you can tell how much a dev cares about their game by the effort they place in their manual, if it's four pages of toilet paper like Ubisoft and Activision games then the first impression you get of the game isn't a positive one. Alot of people here may not remember it but manuals used to be semi novels of content and include all the information about the in-game history along with some related information for real life, such as red orchestra teaching you about submarines or Age of Empires teaching you history. Spending those moments reading the manual on the toilet or browsing it while the game installs are some of the best ways to get you in the mood of the game before it starts.
Bethesda does pretty well with their manuals by today's standards, full colour, at least twenty pages, lore related information and pictures but unfortunately there isn't a notes section the one key page which turns the manual from a tutorial to a useful resource people will keep coming back to. It is the cheapest and easiest page to make for a manual so it has always baffled me why so many devs don't include one but beth if you add that nearly blank page you will have a better manual than 90% of the devs today.
As a fun game for everyone else go compare the amount of detail between your manuals from each year and if you have them pay attention to the amount of detail added or removed for sequels. As a starter take a look at the new vegas manual and then the Fallout 3 manual, if you had never heard of the games after reading those manuals which would you think is less polished?
It's mainly been cut back due to cost cuttings along with of coarse attempting to get better ratings from different organizations like Greenpeace I'd figure.
The manuals were nice, mostly for controls and directions along with an overview of some characters and the main story. But lets face it, majority of us read these on portables and while there are many on consoles who read the manual this has been mostly figured by tutorials at the start or the whole manual somewhere in the menu/ menu gives overview of controls and machanics separately.
Thus while nice the manual isn't needed. While "nerds" have grown accustomed to seeing manuals as a tradition fact is they were mostly to get controls along with the legal documentation into your hands. And placing those inside the disk itself is a better option. I've heard of people writing tips/tricks/etc. Into a given notes section of a manual but I don't see the harm in writing it on a blank oiece of paper you might have.
On the other hand I find a manual or a map like those in GTA IV and RDR in a way did help establish a feeling of quality and craftsmenship in the total package no doubt. But at the end of the day aslong as what's on the disk is up to snuff and there's a readable scanned manual/ or an overview of mechanics in the game there's no need for a manual.
And brushing aside "polish" I hope your not calling New Vegas an inferior game by saying the smaller manual with New Vegas might represent a shortcomming.