It's not about one feature; it's the completeness of the whole browser. Things that you'd expect in Firefox/Chrome in extensions are built in here. It's still as fast as Chrome to the user, never mind silly benchmarks. Give it a spin some time.
Tabs cannot be dragged in and out of windows
Text cannot be selected and dragged into the URL bar
I also fell in love with tab stacking. It's just easier to group tabs into stacks in a single browser window than having separate windows. In later case I spend waaay too much time clicking wrong window.
You can right click the text and select "Go to web address". It's quicker in my opinion.
Another thing that makes me stick to it is the integrated mail client. It's extremely minimal, out of the way if you don't need it. My browser is always open, so having a built in client, that indexes my email, and is searchable even if i'm offline is definitely awesome. I don't need to use Thunderbird, etc, since the minimal features are all i need. Give it a spin!
On the other hand, I do use Opera on my phone. I find tab switching to be much more natural than it is with the default Android browser and I also appreciate the option of proxying traffic through their servers where it is compressed and stripped down to a mobile format. It cuts down on bandwidth and cpu usage which I like, particularly if the data connection is being spotty.
Chrome; very fast, but crashes too often, and uses a LOT of memory (and messes up my Task Manager)
Firefox; lots of interesting add-ons, but slow and memory-hungry (and too often updated, with too many unstable updates)
Opera; very fast, very stable, best with regards to memory use. However, has problems with a few web sites (not compatible, maybe because they haven't tested "leading edge" technologies on Opera ?)