The possibility of a double spend 
But really, this is unpreventable.
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Yes. But Bitcoin itself right now makes double spending on no confirmation transactions extremely hard and expensive. After one confirmation, the probability is close to 0. 51% attacks would be very possible at all times but there is no motive for such an attack and it is too expensive.But really, this is unpreventable.
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For me, probably the transaction confirmation times. That is one the biggest wow or attractive factors with bitcoin for myself personally. It is getting too long.
Yup. If you need it done fast, use dogecoin as an example. I've sent stuff with tx fee, and it took over 6-7 hours....just the time to find a block. etc, etc. A lot of people dont think this is a problem, but its a huge problem.
Using an altcoin is not the solution. I send my transactions always with a 10K satoshi fee. The transactions confirm within 2 blocks. It's that simple. To be even more sure, you must add 1000 satoshi's per KB. Then you are mostly fine.
I know miners need to mine a block first. Adding an appropriate fee will make your chances of getting your transaction included in the next block much higher. If you add an even higher than recommended fee, than your chances of being included in the next block are getting even bigger.