The only thing I hate about the update is Minecarts Derailing, that broke my entire rail system.
Either someone makes a mod that removes derailing or I'll stick to a old version until it gets fixed.
Also hopefully Microsoft fixes things that Mojang never bothered to fix, like Faster updates, maybe even proper mod support and a server browser!
Two main reasons I think.
The first is that, when the game is published, the game's class files are obfuscated, and everyone waits for the Mod Coder Pack to give modders a straight-forward list of what class does what, so modifications can be made accordingly.
The second is just because the architecture of the game changes between updates. As a somewhat obvious example, when the menu was updated in 1.7.*, modders that still had their configuration menus based upon the old configuration had to change it to match the new system.
I basically got used to this, I'm still going to stick to 1.7.10 for a while, that way modders can update their mods and then I'll move to 1.8, however by then something like 1.8.5 is going to be released...so your always in some sort of delay.