Coming second in the European elections was a very bad result for the centre-left across Europe: after years of austerity and the Eurocrisis, and the PES spurring the Spitzenkadidaten process, the S&D group in the Parliament are still 30 seats behind the EPP. However the need for a Grand Coalition to propel Juncker into office has provided an opening for the left, and, like the Grand Coalition in Germany, the essential status quo stance of the biggest party gives the junior partner greater scope for pushing its agenda.
This seems to be the case when it comes to important EU posts. With Juncker as Commission President, there is a domestic German deal to support Martin Schulz as Parliament president for another 2.5 years (with the EPP taking the post for the remaining half of the parliamentary term). Two of the front runners for President of the European Council and the High Representative are Helle Thorning-Schmidt (Danish PM) and Federica Mogherini (Italian Foreign Minister) - to the extent that the EPP are protesting that the PES can't have 3 of the top EU posts. Really, the PES would be wise to go for an economic post over the High Representative position: the presidency of the European Council will be important for negotiations over how the Eurozone will be run, while the PES hasn't a very strong position on European foreign policy (if one of Italy's priorities is immigration, then the Home Affairs Commissioner might be a better position to angle for).
Given the complexity of the EU system, any agreement on policy between the Member States would be hard to stick to, so it's important for the left to win offices that are relevant to their policy goals on the economy. This will be hard to do since it depends on the nominations from the Member States, the negotiations over posts, Juncker's own plans and how the candidates fair in the European Parliament vetting process. There's already plans to revisit the Eurozone budget rules and a deal to soften deficit reduction in return for reforms, so with Italy in the Council driving seat for the rest of the year it appears that the centre-left might have a window of opportunity to push for change.
In the end, however, the lack of agreement over the policy aims of the ECB or even on some form of Transfer Union will mean that the actual scope of opportunity is very narrow indeed. And the broader problems of the centre-left are deep: the fracturing of the old electoral coalition as the economy has become less industrialised, coupled with the decline of two-party politics in most Member States, has left centre-left parties in a difficult position. Uncertain about their domestic support and (to date) largely ineffectual in generating a public debate over a more socially minded Eurozone with greater fiscal firepower, any success over the next few month is in danger of being momentary.
The left in Europe still has a tough task to rebuild itself and shouldn't be seduced into believing that they're in a good position. Europe's centre-right may be more open to some centre-left ideas than you might think at first glance, and they could be well placed to continue capturing centrist and some soft centre-left votes. The centre-left needs to rediscover its voice and identity combined with a platform of reform if it wants to return a a vote - and power - winning force.