At the very core their is no real difference between Origin and Steam, they are both designed to be gaming clients that deliver paid for content, act as a form of DRM and as much as we would hate to think it, especially in the case of Steam, sit there watching what you do so that it can then target advertisement at you with a view to make more cash.
The difference between the two is in the fine detail.
The big issue, when Origin, was announced as a requirement for the installation and usage of BF3, was a section of the EULA that suggested Origin could monitor your computer activity (even if it had nothing to do with EA and their games) and then report that back to EA. This was such a huge issue prior to the launch that it seems to have clouded some more fundamental issues as well as over shadowed a few minor issues.
First the elephant in the room, as far as I can tell Origin does not do anything Steam does not do, by that I mean they both sit there doing nothing, until it is needed. Origin doesn't sit there scanning my computer and sending reams of data back to some EA server so it may be in the EULA but just now it isn't in use.
The second elephant in the room and right now it is the one that defines Origin is the linking of forum and Origin accounts and specifically accounts taken (or rather words said) in the forums potentially rendering you unable to play your legally purchased games. As a direct result of this I will never go anywhere near the EA forums because the concept that an over enthusiastic or idiotic admin at the forums has a say on me playing my games... nope sorry that's a no go.
Beyond that the other issues with Origin are minor almost silly yet you question why are they there at all. Steam runs everytime I boot my computer, it does it's thing and until I play a game it requires very little in the way of input.
Origin, several times since I've installed it I have had to re-enter user login details, despite each and everytime I've had to re-enter them I have confirmed that the 'remember me' box is ticked. Last time I had to enter my Steam log in details... over a year ago, last time I had to enter my Origin details, three days ago followed again a day later.
Why do you have to confirm every update of Origin? Steam does the background download and update and all you do is reload Steam. Origin has to be closed, the updater run, the install windows run and loaded (just like you are installing a new program) it then needs to be restarted by the user and then you have to RE-ENTER YOUR LOGIN DETAIL and add to that it sticks a new icon on your desktop and unless you caught it it will set itself to start when your computer boots, irrespective of what ever it was set to prior to the update.
Why does Origin give you an error message that makes it look like you have gotten your Login details and Password wrong when in fact the issue is the Origin servers? How many folk have gone on to do a password recovery when in fact the issue was the Origin servers?
Why are people still calling Origin 'new' it isn't new. EADM as Origin used to be known was launched over 5 years ago, EA worked out that no one actually wanted EADM willingly so they rebranded it and forced anyone buying BF3 to use it. Origin / EADM is half the age Steam is yet the software is still in Beta and is still filled with stupid bugs and issue that, at the end of the day, make Steam a much slicker, easier and less annoying gaming client to use.