Europeans favor Obama over McCain for a few reasons.
First, McCain is Republican, and Republican brand is not exactly a popular one over here. Bush and the religious right totally ruined the good name of the Republican's over here. Media reports about the insane things the religious right have attempted over the last decade, and what Bush has done, pretty much put McCain in a huge negative view by just having the R behind his name.
Europeans also do not like mixing religion and politics. This is why the doings of the religious right in the Republican party often are in the news over here and not exactly positive. If an European politician even attempted some of the stuff the religious right have done over the last decade or more, he or she would be out of politics fast. The crusade by the religious right against homosexuals, and women's rights is a real turn off in most European countries, including many Catholic. Only Ireland and Poland are probably sympathetic. For example, in Denmark, the only party that is against abortion, is not even in the parliament any more and can barely get the signatures to get on the ballot paper each election.
Then there is also the massive coverage of the US elections. This has actually a negative effect, since the issues like 2000 elections are so alien to Europeans that many were in fact shocked over it. How could the country who claims to be the defender of democracy (and was seen as such) have elections that are no better than a 3rd word country? Also stories about what Europeans would see as clear conflict of interest did not help either.... head election official for a state/area being the head of one of candidates election campaigns in the same area.. would simply not be allowed over here. All these major differences in how elections are run in Europe vs the US have had a negative effect on the view of US elections, which were held up as a guiding light for all democracies world wide.
Now that's not saying that our elections are perfect and our politicians are squeaky clean and there is no corruption.
Our elections are better run than the US's but we have universal (country wise) election laws, and not 42616 different ways of doing it, and this means better transparency which in turn means freer and fairer elections.
As for our politicians... they can be as corrupt as the next. Not far from where I live, a local town hall has the dubious record of the most corrupt in all of Europe. Over a decade or more, several 100 million Euros were paid in bribes by companies and individuals for favours, building permits and so on.. in this area alone. Granted it is the Beverly Hills of southern Europe (The King of Saudi Arabia has his summer place there), but still the amount of corruption is mind boggling. 50.000+ homes are illegal and under a demolition order unless they can be made legal. So you all see.. corruption is not an American thing by any measure

But one difference between the US and Europe.. on a national level and even "state" level, the transparency is far better over here, which means corruption at the highest level is harder to get away with.
Also the biggest difference between the US and Europe, and something many Europeans are often shocked about... the money involved in US politics is insane compared to Europe, and we have far better voter turn out...