Expressing complaints

I'm so sorry, but this will never occur / happen again.
I'm sorry, we promise never to make the same mistake again.
I'm really sorry; we'll do our utmost/best not to do the same mistake again.
Positive response to complaints

Sorry there is nothing we can do about it.
I'm afraid, there isn't much we can do about it.
Negative response to complaints

It isn’t very nice.
It’s really not good enough.
I’m dissatisfied by ….
It’s dissatisfying.
Oh no.
What an awful …
It is not as good as I thought.
Expressing dissatisfaction

It’s disappointing that …
It is unacceptable.
This is the limit I will not take anymore of …
Well, this is most unsatisfactory.
… is so boring.
Expressing dissatisfaction