What causes seemingly normal transactions to not be confirmed?

I seem to be having an issue with transactions lately. Here is the most recent one. I paid 0.0004 BTC in fees, and I see there are quite a few inputs, but this seems like a normal transaction and should clear. However, I submitted it through a Multibit wallet about 15 hours ago and it has still not been included in a single block. Two other transactions I attempted recently have also been rejected, not even showing up in blockchain.info. What is the problem?

What, generically, is going wrong here and how can it be prevented?

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