For one thing, as US legal commentators have observed; what we are looking at is not a Constitution. At least, not in the way that the Americans understand it. 

A Constitution is an agreed political document describing the political structure of a country. The EU constitution has some of this, but in the main it is a Declaration of Intent; a manifesto, or a wish-list of all the powers that the EU wants. It is also a Declaration of Independence by the EU, which formally makes itself a new 'country' rather than a supra-national organisation that one chooses to belong to.

The constitution directly builds on the three treaties (Rome, Nice, Maastricht) which are the foundation stones of a Federation of Europe. This is an aim of which European polititians make no secret. British politicians have lied about this since the seventies.

It does NOT simply bind together, nor does it replace the three treaties, nor does it plug loopholes, nor is it a "simple tidying-up exercise". This is a whole new structure, giving the EU a whole new series of legal and political powers. Again, EU politicians make no secret of this. British politicians mainly remain silent on the matter.

Once the constitution is signed, the conditions for withdrawal make it almost impossible for any country to ever leave.

The moment Britain signs the constitution, we lose our voice as a major influence in world affairs. Everything subsumes to the EU cause. All Britain's international and Commonwealth holdings would become EU property.

Once we sign the constitution, we become one voice in twenty-seven or more. How many euro-laws will we succeed in overturning if they are not in Britain's interest? (Come to think of it, how many laws not in Britain's interest have we overturned so far?)

Once we sign the constitution, any law or directive that we fail to take up will become the 'property' of the EU; we've already handed over our coal and steel, our fishing grounds, our agriculture, our county system; we are about to hand over our entire legal system...

Once we sign the constitution, if we find Brussels produces a law we do not like, our Government will have six(!) weeks to produce a reasoned document of protest to the European Commission. 

The Commission's only obligation is to read it.

Once we sign the constitution, we hand our financial controls to the EU. Our economy would be wrecked by foreign decisions we would never control. Losing the Pound sterling would be inevitable. Joining the Euro would simply be a formality.

Once we sign the constitution, we might as well tell the House of Commons to collect their hats and go, since our MP's would genuinely become rubber-stampers for the EU.