There will just be raid achievements, not 10- vs. 25-player versions in most cases. The achievement won’t care if you complete it in 10s or 25s. If we do meta-achievement mounts, it’s possible we’d still have different colors of mounts, or maybe even different mounts; but for some players that might mean that 25s feels mandatory again, which would be a potential problem. This is the kind of thing we’re going to have to consider carefully, and again, we might try a few different implementations before sticking with something we like.
We’re also not sure about realm first achievements or titles. We don’t want to encourage, say, 25-player focused guilds to run a 10-player raid instead because they think that will get them the ream first title faster. One potential solution is you can earn a realm first title in 10 or 25, but not both. These types of achievements also serve as great content for guild achievements.