Here's my device and preview:
Nokia Lumia 520
WP 8.1 Developer Preview (8.10.14157.200) (Lumia Black update)
And here it goes.
First, let me say I haven't used Opera Mini since v4 or 5 (old Symbian phones) since I prefer Opera Mobile so I had to install it on my Android phone to make a side by side comparison first.
Till a few hours ago, Opera worked fine. It loads fast (I didn't even knew it has a loading screen before reading in the comments) and it loads pages fast. The data savings works great (got a 88% savings so far) but unfortunately it seams to be crashing now on eBanking websites I use and not only. Also, be aware that Opera Mini isn't the only browser that has data savings. UCBrowser does this (I'll probably use UCB as a comparison point since it's considered the best 3rd party browser in WPStore)
*The interface is UGLY. It simply does not fit on WP. I understand the fact that you need to keep the same UI on all platforms but WP is different in terms of UI and so are the apps available for it. You managed to create a simple and clean interface on Opera Mobile. I don't see why you can't do it with Opera Mini.
*Fix the font rendering issue. Fonts are horrible and make reading stuff a nightmare. At least on low end devices.
*The search bar is useless. It should be integrated within the address bar to save space and make it look and feel more clean.
*You are missing Bing. Yeah, 80% of the market uses Google but some might want Bing for various reasons. And given the Windows Live account and Cortana integration, I'd say many do.
*Ditch the title bar. It uses up space for nothing. We all know we're using Opera Mini.
*Add adjustable speed dials, just like Live Tiles are. It's the WP trademark feature and you are missing it. UCBrowser has it.
*Add posibility to select between Mobile and Desktop version of website (User agent on Opera Mobile). I don't see why this wouldn't be possible on Opera Mini. Some websites are 'unviewable'/unusable without a mobile version.
*Proper download section with SD support
*(obviously, the less important thing) Transparent tile
*NOTE TO DEVS - Bring Opera Mobile to Windows Phone
That's it so far.