Among the 18 states with a recall, Wisconsin has one of the hardest signature requirements to get a recall on the ballot.
First, it requires a fairly high number of signatures (25% of the vote for the governor's office in the last election). It could be higher -- for example, Louisiana requires the signatures of 33 1/3 of all the eligible voters in the district -- but it is clearly a healthy amount. Most states appear to use voter turnout, rather than the much higher eligible voter requirement.
I should be a huge caveat to that number, though. Any eligible voter can sign the petition, not just any registered voter. This gives petitioners a larger base to choose from, and also makes challenges to signatures harder to win (petition challenges would be more expensive, as they have to search beyond the voter rolls).
However, the signature requirement is combined with an extremely tight deadline to hand them -- 60 day. Only four states limit the gathering period to 60 days. Most of the others fall between 90 and 180. That the Democrats were able to get the signatures (presumably they feel confident that the signatures are valid, and since they don't live in NY, there are probably less hurdles) to recall Senator Dan Kapanke in half the compressed time frame is clearly a strong sign for them.
Here is a list of next steps on Wisconsin