For me it is transaction times, and the inconsistency thereof. Sometimes confirmations are quick, other times they are not. I would like them to be predictable based on what your transaction fee is. I think that this randomness is a barrier to wider spread adoption.
Have to agree with the above as I also have some transactions taking 10 odd minutes and others that wait for close on an hour for a confirmation. Would be nice if there was a way to sort of 'predict' confirmation times beforehand by for example using a certain fee. So for example if I include a fee of say 0.00001 I know that confirmation time will be 30-60 minutes but when including a fee of say 0.001 it will be around 10-15 minutes etc...
Currently it's all over the place in regards to confirmation times.