I think there are two ways in which Christian worship can fail: one is that it fails to approach God properly (so it breaks the first great commandment); the second is that it fails to enable the community to approach God properly (so it breaks the second great commandment).
Worship can be oriented correctly to God, yet not engage the worshipping community - then it is no longer worship but performance. On the other hand, worship can engage the community but not be oriented to God - that is simply self-congratulation.
I believe there is a 'sweet spot of the Spirit' where a community is enabled to worship God fully; where excellent worship is offered and where everyone shares in this process. Most of all, I believe that as the community grows and develops, so too does the sweet spot migrate and the community as a whole must change with it. The Spirit blows where it will.
To say that 'everyone shares' is not to say that all things are done by all people. There are all sorts of ways in which elements of worship are carried out 'on behalf of' the worshipping community as a whole. For example, when one person reads the intercessions, or one person reads the Scriptures, or one person recites the Eucharistic prayer. Such actions do not necessarily fail to engage with the community as a whole. It is the same with music: there are ways in which the sung elements of a service can be undertaken so as to alienate the community or to enable a sharing in what is being offered (this is why the singing of an anthem as such is not problematic). This pursuit of excellence must, however, be tempered by the element of service. The community has to be carried along together.
There must also be an element of transcendence involved - some element of challenge and invitation to spiritual growth. This transcendence can be found throughout the service - in the set prayers of liturgy, in the sermon, in the intercessions, in the sacrament. It can also be found in the music, in general hymnody or in choral pieces (it is particularly important for choral pieces to be beautiful). Where this element of transcendence is absent then there is no worship as such - we are in the realm of football stadiums, rock concerts and Nuremberg rallies. These can be uplifting experiences which unite and solidify a community - but they do not on their own bring that community closer to God.
The aim in worship is excellence, that what is offered up to God is the best that it can be. This is not always easy, and sometimes, with the best of intentions, worship ends up being a more or less glorious failure, which brings me to Greenbelt, the prompt for this series as a whole.
Other posts in this series:
What makes worship distinctively Christian
Worship is useless