Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
For those of you that haven't upgraded to iOS 8 yet and still experiencing this issue the following was posted over on Reddit.com today:
Easy fix for not being able to update apps through AppStore. (self.jailbreak)

  1. If you have issues where Update button opens the app.
  2. If after syncing through iTunes still shows updates available and icon switches between Update ---> Open.
Resolution: Switch your system language, then change it back. Should restore functionality.​
There's more information in the comments just in case you may need more assistance.

Not working for me.
 
Due to the impending 8.1.1 update killing the jailbreak, I went ahead and updated my iPad air to 8.1 a couple days ago, so the problem fixed itself, but with auto updates turned on, it definitely did do the updates, even though they showed Update beside them still in the list. So, if you want to stay on 7.xx, that seems like your best option if the language thing does not work.

8.1 seems to be working fine on the Air though.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.