For starters, I really recommend reading through my post on what 10-strict raiding is like. It's long, it's in-depth, and it's as accurate as I can make it. It has far more detail than this post.
Raid Difficulty vs Gear:
If you're used to thinking of tens as easier because you were wearing 25-man gear, think again. 10s were balanced for 10-man gear in WotLK. Some food for thought: no ten-strict guild has yet downed Lich King hardmode, yet. Still. All these months. If you want to think it's from lack of skill, think again, and pull your head out of the sand. It's a matter of that slight bit more health, damage, and healing/shielding output: it does make a significant difference in survival and length of the phases.
It's not going to be a walk in the park in Cataclysm. Don't go to 10s just because you think the content will be easier.
Raiding Roster and Attendance:
The average raiding roster for a 25-man is about 35 people: 2 extra players per party in the raid. My own guild uses a roster of 14, which is the exact same ratio, except has less room to ratio between an extra tank or healer vs dps (that's where offspecs come in!).
There will still be attendance problems. Some raiders will stop raiding and you'll have to recruit a new one in their place. Some will have family emergencies, etc. And each one of them that has this crop up will be that much of your raid disappearing. It's about proportional. We've had to kill the Lich King with 9 raiders before; my guild also earned Less is More in Naxx very, very quickly.
When you drop your raiding roster down for 10's, your attendance problems will not disappear: instead, you'll have to pug up a player or run undermanned, and won't have an option to split up the raid and do separate 10's.
Guess what? That PvP offspec may have to go the way of the dodo in a 10. With fewer people in the raid, flexibility for raid roles (tank, healer, dps) and even particular spec strengths (replenishment, AoE, magic type) comes down to each player having a PvE offspec... and they need to be able to play it well, and gear it, and use it. This means pulling a tank into a healing role, something significantly different than what they're used to. Or a healer into a tank role. Or DPS. If your class has more than one role it can play, be ready to get to know one of them very intimately.**
Every class has a different PvE spec they can set up as an offspec, and many of them require, at the least, different gear stats to maximize and a new set of abilities to learn how to use. For example:
- A warlock? Afflic and Destro are your two trees (as far as WotLK is concerned), be ready to swap it around and know how to rock out with both.
- Mage? Hmm replenishment may come into play, some times, or mana-management, or AoE from ze bombs.
- A priest normally holy with a DPS offspec? What if a fight suddenly demands disc?
- A resto druid who's scared of playing feathers? Get over it, starfall is OP (yes, this one's aimed straight at myself!)
- Rogue: personal single-target performance, or AoE love with a nice dps raid buff... or the best spec for dpsing with anesthetic poison?
A 25-man raider is more likely to get away with a single spec in their raids. In progressive 10's? Not so much. You need to know all the PvE aspects of your class... and you may not have another person of your class in your guild to look to for help.
Even aside from specs, hybrid roles need to be observed to maximize the raid's effectiveness, to make up for the smaller number of players in each role. Your DPSers need to help cleanse. Your ele shaman or shadow priest will need to suddenly swap to healing mid-fight as a healer dies or is ported into another plane of existence. Your cat will need to bear it up to take on some adds that are otherwise running rampant while your moonkin pulls off a battle-res on the tank. Your resto druid might need to cyclone something, or warstomp.
There's more offspec, offrole weight on the shoulders of each player in a 10m.
**EDIT: Erick's comment did remind me to add that some 10-man guilds might not care as much about hardcore progression, and so they might not require all of their players to forgo their pvp offspecs in favor of progression in pve. If this is a potential issue for your guild, I recommend addressing it before it causes too much strife as different players have differing opinions on how hardcore-PvE the guild should be.
**Along the same lines: there are some pvp specs that, while not absolutely optimal, could manage in a pve setting, depending on what talents were skipped in favor of pvp ones.
You've likely heard the argument by now: "there's no way to balance the content exactly equally." To the nitpicky extents, this is true. We see it in the fact that no 10-strict guild has downed hardmode LK10, yet 25m has downed hardmode LK25. Could it be balanced, by lessening the severity of the fight? Sure... but after the fact. It'd take far too much time in testing/development to fully and equally balance the fights before live release to be worth it, monetarily.
I really hope there will be a separate achievement tracking system for 10s and for 25s. Separate ranking systems. The 25s stand on their own; the 10s stand on their own. Otherwise each dungeon tier may be dominated one way or the other due to slight discrepancies in balance, to the point that the most hardcore guilds have to swap between 10s and 25s regularly to strive for those world-firsts. You can imagine the logistical nightmare.
Don't go into tens expecting to get achievements easier. It's going to swing both ways. Some fights will be easier on 25, some will be easier on 10. It's the same way, now. Things can be balanced closely, but never 100%, and when it comes to world-firsts, that matters.
Choose carefully when looking ahead for your raid size format. Don't pick it because the grass looks greener: pick it because you LIKE the raid size itself, from its social aspects. Pick it because you want that weight on your shoulders as a 10-man raider requiring more flexibility or you want to be a face in the crowd that plays one spec; pick it because you like the intimacy of a smaller group, or because you want to be raiding with an army.