This website explains exactly what Halal meat is http://www.halalcateringco.com/halal_meat.asp
However it contains a number of inaccuracies and glosses over the truth a bit. The bit about 'carrion' is crucial - it means that animals cannot be stunned prior to slaughter, as technically stunning (when done properly) stops the heart, and the meat would count as dead. So animals have their throats slit consciously. From this website...
'When carried out correctly the sudden drop in blood pressure to the brain renders the animal brain dead within seconds and many researchers have found Dhabiha to be less stressful and painful to the animal than modern western methods of slaughter.'
This is an extra blood supply that runs along the spine and supplies the neck. You can research the details if you like, but this quirk of anatomy basically means that if you slit a cow's throat (i.e. sever it's carotid arteries), instead of leading to almost instantaneous unconsciousness (as it would in humans) this extra blood supply enables the cow to remain conscious for anything from 19 seconds to 120 seconds - which I would suggest is unacceptable. Some studies have shown brain activity 680 seconds (11 minutes!) after the throat was cut.
The extreme sharpness of the blade (called a 'shechita' in Jewish ritual slaughter, but is effectively the same blade used in Halal slaughter ) is also a problem - it's so sharp, and produces such a clean cut, that the carotid arteries retract and clot in some cases, preserving blood pressure, and thus consciousness. You'll notice my quote from the first website doesn't reference the 'research' that it mentions.
This 'ballooning' occurs even in animals that don't have the verterbral circulation, such as sheep and chickens, unfortunately allowing them to experience suffering as well - some studies suggest it may happen up up to 62.5% of cases.
The wikipedia page on shecitas outlines these problems http://en.wikipedia.org/wiki/Shechita if you need a reference.
This page http://en.wikipedia.org/wiki/Zabiha explains the minimal differences between Jewish and Islamic slaughter. Both effectively cause the same suffering, except in one case the cow is facing Mecca when it has its throat slit.
I'm hoping that's a fairly unbiased look at ritual slaughter, but I think you can guess which side I come down upon...
Not a cheery blog post, but some points worth making, I felt!