On the other end of the spectrum, 16 man raid groups are complaining that they aren't being given special rewards for their extra efforts on their harder content (not necessarily my opinion, but one I have seen).
The "special reward" is supposed to be a greater diffusion of risk (1 person screwing up isn't as big of a deal as it is in 8 man) coupled with a larger and, as such, more normalized loot table (doesn't mean much on the token bosses, but it has a substantial impact on ears, implants, non-set items for specific mods/enhs/etc). The problem also exists insofar as the difficulty of 16m v. 8m varies from one piece of content to the next: back at release, 16m was generally considered to be easier than 8m from my recollections (I swapped between a guild that did 16m and 8m). The "16m is hard enough to justify greater rewards" argument has only really been present with the advent of NiM EC.
I recall back when Nightmare Pilgrim was released there was a fair bit of agitation going about concerning a piece of content that was 16m exclusive, which is even more annoying when you consider that 16m uses a different role ratio than 8m so you can't even combine 2 8m groups to create a 16m group without a tank swapping to DPS (16m is generally 3 tanks, 4 heals, and 9 DPS; 8m is 2 tanks, 2 heals, 4 dps). When it's something as simple as a world boss alone, I don't mind (which is why I didn't mind the whole "Dreadtooth at 3+ stacks is a 16m boss because it was just an effective way of creating a single boss that worked for multiple group sizes), but when it's explicit benefits such as secret boss fights and unique titles, it starts getting into the realm of "if you're not in a 16m guild, you're screwed out of content because you don't like dealing with massive numbers of people". It's hard enough finding a decent group of 8 players that I don't mind socializing with; forcing me to get 16 people to get a title and see a boss fight is simply stating that 8m content is, by design, inferior to 16m.