One other thing should not be forgotten, however. The CDU, CSU, FDP have a majority in the Bundesrat. Which means they have and continue to block any Bundes(Federal)-legislation where the Laender (States) have a say. Pension and health-insurance are two of those things(I think). (This is part of the complicated relationship between the Federal government and the State government. The second chamber in Germany is made up out of representatives of the States and there is tough negotiation going on. They don't always vote along party lines, but most of the time they do. When the CDU won North Rhine Westfalia earlier this year, they cemented their already existing grip on this body.
It was then, that Schroeder made his thoughts for elections public and set the steps in motion, that lead to Koehlers decision to dissolve the parliament. The definite loss of power in the Bundesrat cannot be underestimated and is (in my opinion) the main reason, why Schroeder called the election. This has of course also consequences for the future. Any government without CDU participation will, apart from the "moral" win, have no greater power to reform than the current government. And will have to fight the same battles as the once that lead Schroeder to "resign", well the call for the motion of no-confidence.
However, when Schröder came to power in 1998, the SPD had a strong majority in the Bundesrat and had used this power-advantage before the 1998 election as well. It was only in 2001, I think, that the tide changed when Koch in the state Hessen exploited xenophobic tendencies in the conservative base and became Prime Minister. He (and the entire CDU) promised to block the new proposed citizenship law, by using a disgusting signature-gathering process to vitalize his base.
Although I'm for more direct democracy, I say disgusting, because the other side had no say in this, and he used one state to force the CDU's views onto the whole republic.
