So is it a good or bad idea? If it means each individual national parliament being able to block EU legislation, then obviously it's ridiculous. You cannot have national vetos for everything - the Single Market Act introduced qualified majority voting to make the project realistic in the first place. Indeed, the poor quality of the legislation from that period, with Member States adding on caveats, exceptions and exemptions to nearly everything means that it's practically much better that the EU operates in a more open and parliamentary manner than before.
Going back to national vetos will increase the emphasis on backroom deals and lead to poor legislation drafted more by diplomats than democrats. But I don't think that's what Hague was going for (after all, all his talk of furthering the single market would flounder in the face of 28 national vetos); it's more likely that he meant that it would take a number of national parliaments to block draft legislation. I both agree and disagree with Hague on the Red Card idea.
Here's a bit of what he said in his speech:
"The European Parliament plays an important role in holding European institutions to account. It can play a very positive role, as it has along with Commissioner Damanaki in the current reforms to the Common Fisheries Policy. But if the European Parliament were the answer to the question of democratic legitimacy we wouldn’t still be asking it
I think instead that the solution lies in promoting the role of national institutions in European decision-making – because ultimately it is national governments and national parliaments that are accountable to our electorates. They are the democratic levers voters know how to pull. I want to offer some thoughts on how we might do that in a moment
This idea of the right balance between national and European decision-making, and respect for the concepts of proportionality and subsidiarity, brings me to my third key challenge. How can we build a European Union that acknowledges and respects the diversity of its Member States? One that recognises that our national approaches to and ambitions for the European Union may sometimes differ?
We should explore whether the yellow card provision could be strengthened or extended to give our parliaments the right to ask the Commission to start again where legislation is too intrusive, and fails the proportionality test. And we should think about going further still and consider a red card to give national parliaments the right to block legislation that need not be agreed at the European level."
First of all, I disagree with him that strengthening national parliaments is the only way that democratic legitimacy can be increased (bear with me, I will get to the "agree" point eventually). The Eurozone crisis is a key example of this, with political battles going on within national parliaments, but little pan-European structures in which to express these political differences, debate them, and come to a majority approach while honing the policy to take account of criticism, you end up with an incoherent approach.
For the Cypriot bail-out, it was a clash of national parliaments and national leaders. Much of the resentment can be found in the ad hoc nature of the bail-outs, and how the battles are fought along national battle-lines, with little opportunity to be heard on an equal stage, and with no equal and systemic policy that applies fairly to all. This feeling of political inequality, the lack of political leadership at a European level, and the inability to properly debate and influence the central policy is the fundamental basis of disillusionment with the EU (at least in the Eurozone).
You cannot fix the democratic issues with the EU by adding more institutional complexity and blocking mechanisms. To have political accountability, you need a political space that you can influence to create policy that is accountable to voters. The European Parliament and the Commission, if tied closely to the Parliament, would be the best place to start, because it would give a focal point for debating and making policy that can be influenced via direct elections (even with the Council and national parliaments having checks on them).
Because greater instituional complexity and more blocking mechanisms in the system really leads to greater inefficiency and an inability to reach decisions. If the EU cannot make decisions, then that would lead to even greater disillusionment. (And the next European elections can be a good starting point for debating and influencing the big issues with the EU today - see this article by Simon Hix and Christophe Crombez).
That said, increasing the power of the national parliaments can increase democratic legitimacy in the EU when it comes to "...build[ing] a European Union that acknowledges and respects the diversity of its Member States...". At the moment subsidiarity is hard to define - not the idea that power should be exercised as closely to people as possible, but the day-to-day practice of it. The Yellow Card system was a great idea, but it needs reform (a longer time period for national parliaments to object, and national parliaments need to organise greater communication between them to make better use of the procedure).
A "Red Card" system - with, say, a majority or qualified majority of national parliaments being able to block draft legislation - could be a good way of developing a culture of subsidiarity. When the Commission analyses draft laws for subsidiarity and proportionality, it considers the arguments, but there isn't a clear and objective answer to the question, and nor can subsidiarity be effectively defined in a day-to-day way by the courts. There needs to be a political culture of subsidiarity, based on an understanding between national parliaments (and citizens) and the Commission (and the rest of the Union legislature) about what should be done at the EU level and what should be done at a national or more local level. This can really only be done through political debate and contest, and a Red Card system could give national parliaments the tools to start marking that space.
Of course there will be clashes and debates between the EU institutions and the Member States/national parliaments over this - that's the point! But it's a more effective way of building an accepted idea of subsidiarity and decentralisation in the EU over time than relegating the question to impact assessments and wonkish policy documents.
(As a side-note, here's a graphic explaining how EU laws are made [via Kosmopolit]).