Good Restaurant Service vs. Tips

So Saturday night I went on a date to a restaurant I will leave nameless. The date was really nice but the service to me was kind of shabby. Now I am left to ponder why the service was not a little better. Now I know that the restaurant was little crowded but I felt that those around me where getting better service.
I will say that my date and I may have looked like we were younger then what we were and the giggling that that was taking place might have also added to the treatment. (I am not saying that is right). I personally believe that the waiter wrote us off as under 21. Now I did not make a big deal about certain things that did take place or comments that were made by the waiter because over all I was enjoying my date.
But there were things lacking, when my dates drink got to certain level the waiter never asked did he want it to be refilled however the girls at another table he was serving got refills about 3 times. Now for me this lack of attention to the table surpassed the wise comment made about me mistakenly hitting a coaster off the table. He also didn't clear the table off like he should have, being that we were done with appetizers and also did not promote the restaurant well.
Now the bill comes and the tip is already including, this is a result of time when we sat down to eat. But the question I ponder is what could the tip have been if we made our own choice.