I had the same issue.
My iphone 5 on version 7.1.1 was working all OK for updates but my ipad Air on ver 7.0.4 suddenly stopped.
Issue was same what everyone has pointed out already... no update all ...when you press update it just opens up the app.
For the last two months I waited and waited thinking that it might be a app store glitch and will be resolved but it stayed the same...
I know there are workarounds where you can search a particular app and then update it individually but I don't like workarounds... why should I depend on a workaround when I have paid good amount of money to buy an ipad?
Long story short....I fixed the issue by updating my ipad Air to 8.1.2. App updates are working as normal and my ipad is running smooth. I was a bit reluctant to update iOS but after reading different reviews I found that 8.1.2 is quite stable on ipad. I would suggest anyone having this update issue to upgrade to 8.1.2. I think this is what Apple wanted...if you are on 7.0.x then you will likely have this update issue. My iphone on 7.1.1 does not have this issue. Not sure if the appstore architecture changed since launch of iOS 8 or Apple just wanted everyone to use its new iOS .. I don't know...my 2 cents on this issue is to go for upgrade to 8.1.2.