With the recent announcement of the highly coveted Invincible mount, and the subsequent disappointment after finding out that it only drops from Heroic 25-man, I decided to throw my hat in the ring with my own opinions on the matter and pose some possible solutions.
Reading over this thread (and ignoring the requisite trolling that always accompanies posts like that) there seems to be five main issues that 10-man raiders have brought up in regard to the difference in philosophy between the two raid sizes. It's probably worth breaking them down so that they can be addressed separately rather than as one big lump.
1. Legendaries are only available in 25-mans.
While unfortunate, this decision does make sense. Short of sharing lockouts between 10- and 25-mans (which Blizzard seems reluctant to do), there isn't a really easy way to solve this without allowing for 25-man guilds to just run 10-mans in order to get them. This really doesn't strike me as a huge deal. Legendaries tend to represent a group effort more than anything, and 25 people putting one together does seem a bit more "legendary". My only real complaint is that in many cases legendaries tend to have some lore attached to them, which would be neat to experience in some way in 10-man content as well. Quel'Delar was a pretty cool step towards that type of lore-based weapon for the masses, and hopefully one that Blizzard continues.
2. Patterns and crafting materials don't drop in Normal 10-mans.
This argument really doesn't make much sense to me. These patterns are for items that are the same ilevel as Normal 25-man and Heroic 10-man gear. They drop regularly in the Heroic 10-man version of instances and that seems appropriate. In ICC they even made the patterns and materials immediately available through rep or badges. I imagine Primordial Saronite will drop from Heroic 10-man bosses as well to help expedite the crafting process.
3. Cool aesthetic items are more readily available in 25-mans.
This is the issue that really sparked this discussion, since Invincible falls into this category. The main argument is that in ToC, there was a similar reward for completing Tribute to Insanity in either 10-man or 25-man Heroic. A similar suggestion has been made to allow guilds that complete Heroic 10-man ICC in appropriate gear to also have a shot at the mount, which seems like a reasonable request. The main thing I could foresee being an issue in this regard is that it may not actually be technically feasible. The ToGC rewards were based on the attempt counter, and the ilevel based achievements (Herald of the Titan and Tribute to Dedicated Insanity) are a one-and-done deal. However, assuming that Blizzard is able to track the ilevel of the raid on every kill, I don't understand why a similar reward for dedicated 10-man raiders could not be implemented. While this is hardly a new development (since the Mimiron’s Head mount from Yogg+0 was also 25-man only), I think the reason it become a bigger complaint is primarily due to the lore association of Invincible. Blizzard dangled him in front of the player-base with a small story signifying his importance to the lore and then said that he was only available to one set of its progression-oriented raiders. It's hard to not feel slighted in that context.
4. 25-man guilds use 10-mans for more badges and extra practice.
This is bit of a non-issue for me. I agree with Blizzard’s stance on keeping raiding options open for players. If 25-man raiders want to kick back and do some 10-mans in their free time, go for it. Considering that the badges are used primarily for ilevel 264 gear, I don’t see any issue with them getting it quicker. We’ll eventually be swimming in badges with nothing to spend them on in a few months anyway. The practice element is more of a result of Blizzard’s insistence on using attempt counters than anything else. Again, I don’t see much issue with it. It’s not any different than using alts to do the same thing, so it’s hardly an issue that would be solved by sharing lockouts between the raids anyway.
5. 10-man itemization is less interesting and has holes.
This one actually seems somewhat straightforward to address without resulting in any more work for the item developers than their current system. One of the most innovative things Blizzard did with the last two tiers is develop one set of items for each dungeon and then just create a second "Heroic" version with higher ilevel stats. It's a fantastic idea to allow them to more easily incorporate full hard-mode rewards into their design. The main thing I don't quite understand and would seem to solve a lot of problems is that instead of making a 10-man set of drops and a 25-man set of drops why they couldn't just make a Normal set of drops and a Heroic set of drops.
For example: rather than ilvl 251 Abracadaver dropping from 10-man Festergut and ilvl 264 Abracadaver Heroic dropping from Heroic 10-man Festergut, the ilvl 251 would drop from Normal 10-man Festergut and the ilvl 264 version would drop from Normal 25-man Festergut. Similarly, Heroic 10-man and Heroic 25-man Festergut would share a loot table with differing ilevel drops.
Doing so, the ilevel disparity between 10- and 25-mans would continue to exist to maintain the higher reward structure for 25-mans, but there would no longer be holes in itemization existing only in one raiding progression path (usually 10-man) and it would also lower the incentive/necessity of 25-man raiders to run 10-mans for those items that fill the few holes in 25-man itemization (usually trinkets with powerful procs). This also lets Blizzard put the "neat" items (like weapons with procs) as Heroic rewards that would be accessible to both progression paths but in differing power.
And yet despite all of these “issues”, I still would never consider going back to 25-man raiding. 10-man raiding hits the sweet spot of being able to have enough people that coordination and strategy are still a challenge, while maintaining a strong level of accountability for every raider in the group. To me, there just is no comparison.