Hummus isn't the same without tahini. It adds incredible flavour and texture. If you've had unpleasantly bitter tahini in the past, splash out and get yourself a really good quality one. Look for one made with Ethiopian sesame seeds that have been hulled. Ethiopian sesame seeds are purported to be the best, and hulling the seeds makes the tahini milder. If you still don't want to use it, in a pinch, you could get a similar consistency by using another nut or seed butter. It will give the hummus a very different flavour profile though.