I'd say we were right to oppose war on Iraq, for the following reasons:

1. The stated reasons for going to war were lies.
(a) Weapons of mass destruction
There has been little or no evidence of chemical or biological weapons found. There will be no evidence of nuclear weapons programmes. Even if some weapons are found, Iraq had little capacity to deploy these weapons - they had few missiles to carry the payload. Importantly, even now that Baghdad has fallen, no chemical or biological weapons have been used.
(b) Iraq was a threat to its neighbours.
We can see that the Iraqi army posed no threat to any of its neighbours. It could not stand up to the US army - and in any situation in which Iraq were to attack any of its neighbours, the US would have launched military
action.
(c) Iraq is connected to Al Qaeda.
There have been no links shown to be true.
(d) Iraq will be better with Saddam gone.
Not many in the anti-war movement are not glad that Saddam Hussein's rule is over. But the state of Iraq post-war is not yet apparent. Military government by Jay Garner may not be very free. The long-term democracy of Iraq is still in doubt. Kurds will be especially concerned to be ruled by a US administration when they have had autonomy for 12 years. A look at Afghanistan shows that rebuilding and stability does not seem to be of great concern to the West.
(e) It's not about oil...
Control of oil reserves is essential to the US economy - even if oil revenues are not leached away by the US, the supply of oil is still very important since world oil output is expected to peak by 2012-2020. Reconstruction contracts will be lucrative for US companies close to the administration and control of the Iraqi markets will also be lucrative. The war was also about US "full-spectrum" military dominance.

2. Deaths from military action
(a) Whether by deliberate targetting (which has occurred in many incidences) or by mistargeted weapons or destruction beyond the target zone, civilians have been slaughtered. The web-site www.iraqbodycount.net now stands at 1139-1375 civilian deaths. This is undoubtedly an underestimate, since the figures are collated from Western news agency reports. The figures of injuries will be massive.
(b) Indiscriminate weapons have been used, which is a war crime under the Geneva Convention. Cluster bombs, depleted uranium and high-explosive have been dropped in civilian areas. We will have to wait to see what the long-term deaths are from cluster bombs and depleted uranium.
(c) Iraqi soldiers have been killed. Many Iraqi soldiers are conscripts.
Figures for deaths of soldiers are difficult to estimate - they will be in the many thousands. US/UK estimate is 4000.
(d) US/UK soldiers have died. CNN reports 136 US/UK military deaths.
(e) 9 journalists have been killed, many in 'friendly fire' incidents.

3. Humanitarian situation.
(a) Food and water. There is not enough water in many areas of Iraq. The aid is simply not enough to meet basic needs of the population - and in Umm Qasr, the US is allowing those with tankers to sell water. People will die from thirst, and from disease brought on ny drinking unclean water. Pandemics of cholera etc. are possible.
(b) Chaos. Widespread pillage, which the US and UK are bound to protect against under the Geneva Convention. A Muslim cleric was hacked to death in Najaf. More looting and reprisals are likely.

4. Possibility of escalating military conflict.
(a) Kurdish fighters have occupied Mosul and Kirkuk. Turkey says if they take these cities permanently, it will invade Kurdistan.
(b) Both Iranians and Syrians have been killed by stray fire from Iraq.
The US has begun to threaten attacks upon these countries, and upon Lebanon. Part of the argument against war on Iraq was that this was not the last target on America's list - that argument seems to be solid.
(c) Civil war. Some from Kurdish and Shia communities in Baghdad have been seen to be celebrating. But the Sunni response may not be so jubilant. If the US occupation drags on, increased discontent may break out as military action.

5. Playing Russian roulette is dangerous
Some of the outcomes of the war have been less severe than predictions (e.g. UN estimates of 100,000 direct deaths from miliatary conflict, 1.45 million refugees). But if somebody playing Russian roulette survives the pull of the trigger, I will not then say that my advice against playing was wrong.

6. Risks of terrorism.
(a) Within the Middle East, Westerners will face attacks.
(b) The US military presence in Saudi after Gulf War I helped Al Qaeda recruit. Al Qaeda and other terrorists now have another 'reason' to attack Western targets.

7. T.I.N.A. was not true
T.I.N.A. or 'there is no alternative' was the charge of those who were pro-war. The obvious alternative would be to not go to war, which would have avoided the expected and feared consequences of war. Further, the raising of the sanctions against Iraq would have reduced the civilian deaths from starvation and reduced Saddam Hussein's control on the population making revolt more likely. The introduction of weapons inspectors showed Iraqis that Saddam was not powerful, but indeed he had to comply with UN demands.