Basically, Sassenach, he's right. He's exactly right. Again, you're looking at the formation and previous condition of the Democrats and Republicans. The Grand Old Party did indeed have different roots, formation process, style (and so forth) than their counterparts across the aisle; while the Democrats were a little more machine politics-oriented, geographically diverse and therefore were less ideological. Again, in the past; not so much now. Whether you're blaming hyper-partisanship or polarization of American politics--whatever you care to call it--the effect has been that it has made both parties ideological. Pundits and political science professors alike have commented on this. That was a very good observation of you, but the situation has changed.
I don't buy this. You're too close to the situation to see it for what it really is. Anybody who isn't American and who's halfway clued up on American politics doesn't recognise any kind of definable ideology that they could readily associate with the Democrats. It simply isn't there. Certainly they don't practice anything that could be described as 'progressive' by any definition that makes sense (sorry RJ, but you're flat out wrong on this one). If any European politician tried to propose something like Obamacare they'd be demonised as some kind of right-wing fundamentalist ideologue. On almost every issue the Dems are essentially just the diet version of Republicans. They only differ on the culture wars issues such as abortion, buit even here it's more a matter of rhetoric rather than policy since nobody has really made any material changes to abortion laws since Roe v Wade.
The Republicans are much more ideological than the Dems, have been since Goldwater and are becoming ever moreso. This has led to a culture of extreme partisanship on both sides of the aisle, but it's only one of the parties that is seriously motivated by deep-rooted principles.
That's how it looks to me anyway, and nothing any of you have said so far has convinced me otherwise.