Blizzard has stated before that the 4.0 patch will change Azeroth for everyone, not just people with the expansion. Even if you haven't bought Cataclysm yet when it's finally released, you'll still see the changed world. You just won't be able to roll a worgen or goblin, go to the new zones (Uldum, etc.), or take part in Archaeology.
"Do you know how Cataclysm will affect Loremaster? I got within 400 quests and put it aside to finish later, but do I really have a limited amount of time left?"
We don't know specifically how it'll be affected, but Blizzard has said that we needn't worry about it. We can extrapolate a few things from precedent, though:
1. Blizzard has never removed titles after an achievement becomes obselete, so we can assume that people who have gained Loremaster will keep it.
2. The Loremaster quest count has been updated before, so we can assume that it will be again.
My personal guess is that we'll get a Grand Loremaster type of title, either for completing the new Loremaster or for being a former Loremaster. Loremaster may well become a Feat of Strength.
"Would they reduce the costs of frost emblem gear? I somehow remember toward the end of TBC they did something like that. Currently I don't raid much and it would be forever until getting some 251/264 gears. Even with ICC raid, that would still take quite some time."
It's likely that, if you do any raiding at all, you'll be swimming in Frost badges by the time Cataclysm hits. It's possible that they'll follow the BC pattern and make 4.0 a Frostapalooza, but I honestly don't think they will. With only five new levels, they'll likely want a lower item level baseline for leveling to 85. I'm sure that as 4.0 grows closer, we'll hear more about their plans.
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