An ally should be someone who has your back. Who understands your history and walks beside you in good times and bad. The relationships between many gay men and lesbians during the early years of HIV/AIDS are good examples of allies in action.
Though it’s not always the case nowadays a queer ally is usually a middle-class (invaribly white) well-educated individual taking it upon themselves to speak for what they see as the silenced, oppressed and vulnerable sections of the queer communities. The irony that the people they are usually speaking for may not identify as queer nor that they are further silencing them by talking for them is usually lost on them. The further irony that, once again, white middle-class people are taking charge and passing judgement is also lost.
A queer ally should be something that you are named – not a label you give yourself.
If you seek to be a queer ally it should be a set of practices that you aim to follow – not a set of rules you primarily judge others by.