What's happening with Loremaster? Do I have to start all over again if I don't already have it?
Nope. In a nutshell, whatever you've already done isn't taken away from you, but how much you have left to do is subject to change. If you already have one or more of the meta-achievements, you'll still have them post-patch (e.g., Loremaster of Kalimdor, Loremaster of Eastern Kingdoms). If you've done, say, 529 quests toward Loremaster of the Eastern Kingdoms, you'll have those 529 quests credited to you, but the number of quests you have to finish for each achievement has probably changed. However, that's where things get hairy, as you won't have credit for any of the new quests introduced, but you still won't be starting from scratch.
A Horde character before the patch needs 685 quests for Loremaster of Kalimdor and 550 quests for Loremaster of Eastern Kingdoms. An Alliance character needs 700 quests for Loremaster of Kalimdor and 700 quests for Loremaster of Eastern Kingdoms.
Post-patch, that same Horde character will need 624 quests for Loremaster of Kalimdor (split across 13 zones, each with its own achievement) and 603 quests for Loremaster of Eastern Kingdoms (split across 14 zones, each with its own achievement). The Alliance character will need 714 quests for Loremaster of Kalimdor (split across 13 zones, each with its own achievement) and 648 quests for Loremaster of Eastern Kingdoms (split across 14 zones, each with its own achievement). The Outland and Northrend quest requirements appear to be unchanged for the moment. With the Cataclysm zones becoming a requirement for Loremaster, it's possible that the title may be briefly unattainable by players between patch 4.0.3a (if all of these changes go live then) and Cataclysm's release.