In an earlier post I asserted that the Charter of Fundamental Rights will increase the protection of workers' rights in EU law, and Owen has written a great article on the misinterpretation of past cases. If the Lisbon Treaty is rejected, then the position of workers' rights in EU law stays the same.
So it's hard to see exactly how workers' rights can be used as a reason to reject the treaty: the No campaigners on the left have yet to produce a convincing alternative, and it's hard to see how the political shape of the EU and the member states could support their demands.
One of the biggest changes the Lisbon Treaty makes, is to enshrine the Charter of Fundamental Rights as part of Treaty law - you can find it here (PDF). It's based on the European Convention of Human Rights (which is the product of the Council of Europe, which is separate from the EU), which the EU would also sign up to if Lisbon is ratified. Though the European Court of Justice has drawn on the ECHR as being part of the legal culture of Europe, this would be the first time that these rights will be part of the EU's primary law, along with the four freedoms (of goods, services, people and capital) that underpin the single market.
The aim of enshrining these rights is to boost the social side of the EU, and to make its laws more responsive to human rights and workers' rights. Though the EU has long had a positive impact on gender equality and other rights, the Charter is a response to the criticism that the economic rather than human elements of the single market dominate the EU's law making. The Charter will only apply to EU law, and national laws that implement EU laws - so it can't be used to expand the EU's powers or competences (it even says so explicitly in Article 51(2)).