Classic/TBC: different 10/20 and 25/40 instances. Best guilds were poaching players from less progressed guilds, less progressed guilds were stuck on infinite circle of gearing up players just to see them move on when they got shinies.
WoTLK: running same instance twice a week. Up to 4 times during ToC patch. Gear resets every patch, making progression meaningless.
Cataclysm: death of 25 man guilds, except for hardcore guilds. Many realms no longer have any 25 man guilds at all. Gear resets every patch, making progression meaningless.
4.3 and MoP: same as Cata + LFR, so people are running same instance twice a week.
Whatever Blizzard does, there will always be unhappy players.
The reason Stormwind park was completely destroyed was because the night elves there had the audacity to tell Khadgar that Boomkins are superior to Mages in every way. Deathwing just happened to be flying by.
It's always been like that.
Also, some encounters are more manageable in 25-man as opposed to their 10-man counterpart.
And, some encounters are more manageable in 10-man as opposed to their 25-man counterpart.
It depends on encounter-design. Encounters that require people to perform various tasks but at the same time have mechanisms that disable some people from performing them occasionally are naturally a greater hassle on 10-man than on 25-man. You have more people to fill in when the primary people temporarily cannot perform that task. On the other hand, you have encounters that require the raid to spread out, and still everyone needs to be in healing range. Way easier to handle for a 10-man.
Communication on TS/vent is also easier for a 10-man. We've had moments where various people were announcing important things... In my former 25-man raids, it was much more quiet with mostly the raid leader and the officers speaking up - unless in cases of emergency. In 25-man, I never felt like I had so much responsibility as a healer as I now do, when I raid 10-man (it sort of reminds me of when way back, I healed LK hc 10-man on an off-night, 2 healed, me being shaman... and nobody in their right mind actually wanted to bring a shaman there, at least not before the buff had stacked up to 30%. It was exhausting. So much depended on me. It was a step up from 25-man there, seriously). If you had asked me six months ago, I'd have said "Me, raiding 10-mans? Go away, 25-mans are way more epic. You'll not see me dead going 10-man with my main." Times change. People change. Old friendships break and new ones form. The raid that you're in now may in a few months no longer be the one that you want to be in, and you might leave and form a new one with people who grew on you.
Oh and, just to be an annoying person, I'll state: "A dps player cannot fathom the levels of difficulty of a raid size." I've raided enough on my dps alts to be able to say that, on most encounters, it made no difference to me if there were 9 or 24 more people in the raid. I was just performing my rotation and pushing Hymn of Hope and the likes when told to do so. When I heal, I feel more of a difference. Some encounters compliment my playstyle and class mechanics more than others, meaning that I will be performing better and more easily on one raid size than I do on the other.
Okay, Stone Guard is an example, where a dps player has to be on their toes about chains and lighting up tiles, but that one's a classic example where having more people to fill in for lighting tiles when the usual people are chained to something unfortunate is an advantage. It's silly to see how 25-mans can light up the entire room with ease. But they get an additional mechanic to deal with, and they have more people that can fail on them (which they shouldn't but... we've all raided enough to know that people don't always do as they should, and that factor is multiplied by 2.5 in the larger raid size when you look at the average, non world top 100 area. Because the people up there fail less, independent of their respective raid format).
I suppose the point of this entire rant here is: Why do we even care? So much of what we're discussing here are entirely subjective impressions! Anecdotal evidence! Differences in setup! Picking the best 10 out of a roster of 30+ people (trust me, I've been there too), and then claiming that it's all so much easier. It's not. It's not all just black and white. There are too many variables to give an objective point of view. Which is why arguments like this are totally pointless. And yet I'm participating in them all the time, if only to counteract all the one-liners that just flame around.
As for this endless discussion about what raid size is the most difficult, it depends on the boss and what abilities the boss uses during the fight as well as what classes/buffs you have in the raid group when you fight them.
Some bosses are harder on 10 man, some are harder on 25 man (an example is that it's easier for 10 players to spread out in the same sized area as 25 players have to use).
To the people wishing for 15 man raids only, I say screw you. As I personally don't want to raid 10 man or 15 man, I would like to keep my 25 man option thank you very much.
Seriously DO run the numbers instead of attempting to make excuses. The extra leeway 10 man gets allows for poorer play, messing up your rotation, missing a few globals, and still meeting the DPS requirements without a sweat.
I dont know why people are so pissed, though i see this as to little to late. Would have been a good move durring cata but meh now. I dont get why people are mad about the item levels, if all your going to do is 10mans then why does 25mans item level affect you at all? No one says you have to do 25mans. 10mans will be perfectly managable with the item level it has now. With the advent of 10man guilds alot of servers became ghost towns with alot of 25man guilds dying and smaller 10man guilds forming but in the process forming a smaller community and alot of times shutting themselfs out. Iv done both 10 and 25 and enjoyed 25 far more, servers were more active and fun durring the wrath days.
And i must say again, NO ONE IS FORCED to raid 25mans.
I wouldn't mind the separate lockouts so much, but better gear just because your guild has more active raiders? No sir, wouldn't like that at all. I already felt like a second class raider during wotlk because 25s got better gear AND were the only ones to get legendaries