The big thing about Origin is that it isn't steam. People have steam, people like steam, steam works really well these days, has really good customer service, and has amazing sales and deals on games. Origin on the other hand is new, sometimes buggy, with rather poor customer service at times, and has none of the awesome sales that steam does (in fact, EA was openly opposed to sales saying that they "devalued the product.") So part of the reason that people don't like Origin is that they aren't getting steam prices for EA's games anymore.
Another thing is that Origin snoops on your computer and collects your information for EA to use. Now steam does this as well, but steam ASKS whether you're ok with that, and you can decline giving steam detailed information about your computer, something that you can't do with Origin. This is of course coming from one of the least trusted companies in America, so you can see why people have problems with it.
And the last thing is that Origin is basically always on DRM. You have to have Origin on in order to validate your game, even if you just want to play the single player portion of it. Of course Origin isn't as bad as always on DRM, because it just validates your game when you start the game, and doesn't require you to have a perfectly stable internet connection at all times, but that still doesn't ingratiate it to people who don't want to have to have an internet connection to play a single player game.
