I've been in a community recently with a decent mix of people who are die hard DMC fans and folks who have never played a DMC game before. I've only played fifteen minutes of the first DMC myself.
The way I see it, if this demo is a good representation of the starting difficulty, and it truly is easy compared to the insanity that is DMC3 (I've heard enough about it), then there's a good explanation for it. In fact, it also goes for why you start with Nero. It's to introduce those who missed the last three titles into the series. Difficulty is easy to see, but let me explain how it's better to have Nero than Dante for the newbies.
Dante is an established character. He has his past, and has his background. It's honestly bad story-telling for him to go over the past three games for the new players, assuming he is the main character. Plus, they'd have to go back to establishing it all over again, annoying the old fans, or just keep going like nothing has changed, making the new fans lost. However, with Nero, you can reintroduce Dante in a way that pleases everyone. The old fans get to figure out what has changed between the past games and now, uncovering some mystery of a favored character, while new players get to gain backstory and learn about the previous lead character.
I'm not irritated by Nero myself. I mean, I think looking almost exactly the same is a bad move on Capcom's part, but when I saw the extended trailer for DMC4 the story elements of Nero got me very interested. It's not like we're going to be seeing another Raiden, as Nero has already proven he can do some pretty hefty damage.
Now, as for the demo itself, I enjoyed it, but I have mixed feelings. It's definitely fun, and I like the ability to juggle enemies. However, after playing a hack and slash like Ninja Gaiden, it feels a bit shallow. I know Ninja Gaiden also came after DMC, but the combat was ultimately more satisfying. Either way, DMC4 is definitely a title I'll be purchasing.