Actually, saying that raids at the same tier reward different gear was slightly misleading of me. In actuality, the 10- and 25-man versions of raids in Wrath are tuned a tier (13 ilvls) apart. In the terminology of prior versions of WoW, if Naxx-10 is a T7 raid, we ought to call Naxx-25 T8. Ulduar-10 would then also be T8, and Uld-25 would be T9. We don't use names like this probably because the "tier" designation is related to tier sets that drop in raids, and the Naxx-25 T7.5 set is obviously more similar to the Naxx-10 T7.0 set than to the Uld-10 T8.0 set.
However, this is mostly a technicality. The take-away point is that in Wrath, the developers seem to have decided that 25-man raiding is better than 10-man raiding. It rewards better gear. And in Ulduar (which is of course the centerpiece of the upcoming 3.1 patch), this difference sharpens, since as far as I can tell there is no way to get the new legendary healer mace, Val'anyr, by doing 10-man raids.
The information thus far is that the Fragments of Val'anyr (of which you combine 30 to form the mace itself, reminiscent of Atiesh) drop mainly from hard-mode 25-man bosses, with a smaller chance to drop from non-hard-mode 25-man bosses. (Please correct me in the comments if you know this to be incorrect.) Which means if you're a healer in a 25-man raiding guild who's willing to put in some effort, you can probably get Val'anyr sooner or later. But if you're in a 10-man raiding guild, simply put, you're flat our of luck (unless you want to do a lot of pugs, I guess).
In my opinion, there is no justification for this. A common argument is that 25-mans are harder, and thus 25-man raiders "deserve" better gear. In my experience, this is absolutely false; the bits of 25-man Naxx that I've done I could practically have slept through, and 10-man Sarth with 3 drakes is generally accepted as the toughest fight in the game right now. This may be different in Ulduar (I haven't tried it on the PTR), but I doubt it - the more raiders you have, the more room for slack.
Another argument is that the 25-mans require more time investment. This is true to the extent that it's simply harder to wrangle 25 AFK-happy, incompatible-schedule-having raiders than 10 of them. But if that alone is enough to command better rewards, every guild leader who's ever held a guild meeting should get free epics. We should be rewarding skill, not suffering. This is related to the argument that 10-man raids are for "casuals", while 25-man raids are for "hardcore" players.
The third major argument in favor of the gear disparity between 10- and 25-mans, and a very telling one, is that if 10s had rewards equal to 25s, nobody would do the 25s. The only reason I can think of for this is that 25s are less fun to organize and/or to play that 10s. But should Blizzard really be using gear to incentivize players to do less fun content?
Note that I'm not advocating for free epics here. Reward should be proportional to effort. But reward should not necessarily be proportional to pain, which seems to be the idea behind the third argument.
I'll cut to the chase here: 25s are no harder than 10s, and not inherently better in any way that I can see. It's time to stop discriminating between the two. Let 10s and 25s reward the exact same quality of gear, and let the players' choice on which raid size to run be motivated by how many of their friends they want to play with, or the size of their guild, or what kind of a raiding environment they like, not the lure of better gear. And as an added bonus, you no longer have to itemize 10s and 25s separately if you don't want to.
If this leads to 25s being virtually abandoned, honestly, I see no problem with that. Let the most fun content win out. You don't see many people doing Heroic Oculus - should we make all drops in it 6 ilvls higher? To me, the answer is an obvious no. Instead, fix the content to make it more fun.
I don't, as it happens, think that changing 10s and 25s to drop the same gear would lead to an abandonment of 25s. There are lots of players out there who like the environment and the action of a 25-man raid, and there are lots of guilds built around that size of group. I seriously doubt everyone will suddenly start doing 10s just because their stats are no longer better than everyone else's.
Incidentally, this provides a solution to a concern about achievements that is currently under debate in the PTR forums. The realm-first achievements for Ulduar-10 were removed under concerns that 25-man guilds would pop into Uld-25 to get a few epics and then go back and clobber Uld-10, reaping the achievements. This is not behavior that Blizz wants to promote, so they pulled the Uld-10 realm firsts. If Uld-10 and Uld-25 rewarded the same gear, this would no longer be a possible strategy.
They are apparently planning to implement an achievement for defeating Algalon without having any gear of greater ilvl than Uld-10 hard modes (226), but we still don't get any realm-firsts in 10-man raiding, while the 25-man raiders do. Yeah, I'm whining a bit, but it is a disparity, and it does give the chance of a greater reward (a realm first) for easier content.
When I was hashing out this argument with the WoW Insider team, Alex and Rossi were very passionate that awarding the same gear from 10s that you do from 25s would kill 25-man raiding, despite the fact that many players prefer 25s. The reason for this would be that 25s take more time (which I'm not disagreeing with), and that players will optimize their play for greatest character advancement per time. In other words, if you can raid Uld-10 in 3 hours for gear of ilvl X, nobody is going to take the 4 hours to raid Uld-25 for gear of the same ilvl, no matter how much more they may prefer the 25-man-raiding environment.
This argument is reasonable, and Ghostcrawler appears to agree with it (or at least agree that the death of 25-man raiding would result from gear parity). However, I just don't buy it. I think that as long as the reward rate disparity isn't huge (as it is with current arenas, leading to their unpopularity), people will play what they want - if they like 25-man raiding, they'll do it. I don't think raiders aren't motivated by gear - they clearly are - but I also don't think they're solely motivated by gear. Other factors are at play.
For my 25-man-raiding constituency, I have make a pair of polls to test my hypothesis. These are obviously very unconclusive and unscientific, but I would appreciate your responses nonetheless. 10-man raiders or non-raiders, please refrain from voting; I'd like to keep these polls as representative as I can.