Liverpool does not have a natural player who can lead the side. Gerrard is a great player, but he does not seem to have a hold over the players commitment. Someone in the likes of Fowler or Gary McAllister would fit this bill. Litmanen does not fit into this bill, but he is a player who tends to do great things in important matches or deliver when the chips are down. It is not surprising that Liverpool started the descent soon after these three players left.
As for the weaker players, it is very easy to point out Biscan, Traore, Diao and sometimes even Cheyrou and Riise, they don't make much of an impact to the team. Diouf on the other hand may not be much of a success as a striker, but he plays very very well down the right flank and his tenacity is an asset to the team, something you would notice in players like Gerrard, Henchoz, Carragher and even Kewell. Heskey should learn to more like them, sometimes watching him play and tumble at the slightest contact makes me wince. The team on the overall simply does not have enough aggression when they play, but talent is abound and there can be no denying this.
As for Westerveld, he is a good stopper of shots, but he can't handle high crosses, and fumbles them just like David James. Think back to the goal that he conceeded to Jordi Cruyff at the last minutes of the UEFA Cup final. He had a look of full concentration when he came for the cross, but still conceeded the equalising goal in the most dramatic fashion.
One thing about Houllier. Players who have been too outspoken, disgruntled or quick to express their own ideas publicly will always get the boot or allowed to leave Liverpool for obvious reasons. Westerveld, Fowler, McAllister, Babbel, Anelka, Ziege, Camara just to name a few that I know of. Players who carry long term injuries for long periods of time will leave as well, Berger and Redknapp are the obvious ones.
Why Litmanen was allowed to leave is a great mystery. Maybe because of his great friendship with Fowler on the field, possibly their reluctance to be outdone by the striking partneship of Owen and Heskey, or maybe it just boils down to dollars and cents.