UA's site is much easier if you buy tickets for others.

After picking your flights, you enter the traveller info, CC, billing address, email address, and you're done.

To do the same at co.com, you have to add their name into your saved travellers, their CC into your payment methods, you have to add their address in your billing addresses, their email to your email addresses, and so on. It's like 5 or 6 extra screens and extremely time consuming. And all this to book a ticket using a CC and address you'll use only once. So now you have a sloppy profile with all this extra junk in there you have to go back to and clean up.

DL's site is also easy for purchases, much like UA. I don't even store CC info at either of these sites, because I don't have to. It only takes 2 seconds to enter it, and I use different cards for different purposes. Meanwhile, co.com makes me store my CC in there.

And of course CO's lack of instant ticketing which has been already been brought up.