mostly bug fixes
mainly the one i like is that they fixed the ghosting tnt
Who says Dinnerbone's fixes are in there? Jeb only said Bukkit. Not that it wouldn't be, actually makes sense, but he didn't mention it... Wait, is Dinnerbone part of Bukkit?
Why do mods break, anyway?
Because of Obfuscation changes, as well as general coding changes. Basically, every big release, the code is changed. They have code that, when compiling the game, all commands and such are renamed sequentially. Basically, the first command that appears in A. Then AA, and so on. But it's differently ordered each time like... Well, I can't explain this right, but basically everything is named differently as well as their own coding changes which break it.
However, minor changes like 1.2.4 shouldn't break most mods since they only change obfuscation in major versions. The only breakages will happen in what they actually changed in the code, but that's a simple fix with obfuscation is already known.