"not sure why this should be such a big deal."
Oh, come on. Seriously?
The Mail has taken a journal entry from when Miliband's dad was 17, and accused him of "hating Britain" (despite the fact that he fought for the UK in WW2 and spent most of his life here). That's quite a serious smear - it's a very nasty thing to say about someone, especially when there's precious little evidence for it.
And what's worse is, even if it were true, the Mail are using this to smear Ed Miliband by proxy. They have so few real arguments, such little intellectual integrity, that this is what they are forced to resorting to. And they are still taken seriously as a source of informaiton.
"don't the papers, news and some on AB go on and on about tory posh boys all coming from well to do families"
This isn't really something I'm comfortable saying myself (because like you I don't think it's really valid) but smearing someone's dead father by putting words in his mouth just so it suits your political agenda against that person is several orders of magnitude worse.