Will start from positive
I really like that new tab is created after currently active one (both on creating new tab and opening links).
I like menu and controls on the right: less hand moving and I don't cover part of the screen with my hand while taping on a buttons. This is on a tablet. I think that menu on the right is not very good idea for phones which are usually hold with just one hand.
I really welcome Private tabs.
Some improvements in in-page search: counts added and ability to jump on previous instance. Nice!
Discover page is nice, but I can't zoom here (body text is small) and content tiles are constantly reloading while I scrolling up and down.
Would be useful if I can select multiple regions for Discovery. I.e. Russia and Ukraine or USA and Canada or Global and USA. Even more useful would be if I can setup content categories for each region individually. I.e. for Global: news, science, technology; for USA: lifestyle, living, entertainment.
Discovery categories icons are rough in small size when they placed on tiles.
Sometimes while navigating back in history web page becomes black. Only switching on other tabs and return to problematic one solves the problem.
Default portrait mode on first start (initialization) and rotating from portrait to landscape on every cold or warm start of the browser.
No explicit Quit command in menu.
Was stumbled for a couple seconds with inability to go back in history (there is no usual Back button). Then, after I kind of used to it, I find out that this soft button is always looks active, but if you have no history to navigate it does nothing. Basically you need to tap it couple times before you understand that you can't go back any further. Maybe it's a good idea for portrait mode on a phone, but is any other case there is plenty of space for normal back button.
No Forward button. If I went back, sometimes I want to return.
*Oh, found it under menu.
On a tablet in landscape mode tab switching is unproductive. Thumbnails are too wide and I can see only 3-4 at once, but there are plenty of space to have 8-10 of portrait or square thumbnails.
Long press on a link that has formatting inside (say, part of a link is in bold) highlighting only part of the link.
Sometimes same text rendered blurry, sometimes doesn't. Try to double tap and zoom with pinch on the same text multiple times to see the effect.
No text wrapping by default (as was in Opera previously), nor there is an option for this (as in Firefox with "Pinch to reflow text" setting), nor text reflow works as in most WebKit-based browsers (pinch, then double tap).
Fonts are serif on many sites (Reddit as prominent example) instead sans serif ones.
Overall I can open new tabs and switch between them in Opera Mobile 12.10 noticeably faster; those operations "feels" reliable and solid-built there. Opera Mobile 14 is not as responsive, and I "feel" some lagginess.
Newly created tabs (after link is opened in new tab) not loading in background and shows blank thumbnails. I expect opened links already loaded in background before I switch to them.
How I can search on something different than Google?
VK.com (vkontakte.ru) is broken somehow. I can't go anywhere from initial Feed page.
How I can create my own searches? I usually have 3-5 custom searches in addition to couple built in options.
Speed Dial thumbnails are usually blurry and unreadable in comparison with Opera 12.10. I prefer to see some site content on thumbnail (preferrable) or a small icon. Most of the sites don't have an icon, and probably never will. Try some: http://d3.ru http://habr.ru http://tema.lj.ru
Bookmark (add to Speed Dial) button sometimes blinking or switching to loading state after page seems to be loaded, so it's become hard to bookmark such page. Maybe this is caused by some AJAX calls (e.g. changing banners, updating content). Example: http://tema.lj.ru
Bookmark (add to Speed Dial) button has no "already bookmarked" state. Even worse that pressing this button again will remove existed bookmark pretty silently (there is tiny message far below from the button itself that is displayed for a short time).
As for bookmarks which are gone. If Opera Link will work in a future, how desktop bookmarks will be synchronized? If bookmarks will be gone in desktop version too, well, in this case I have nothing to say... Maybe Opera Mobile 14 will sync only SpeedDial via Opera Link, and it is disabled for now because it's incompatible with current version of Opera Link.
Page loading indication was much more useful and visible in Opera 12.10 -- it showed progress, and you had some guess of how much is loaded already.
After I played with browser a bit and opened ~10 tabs, and then trying to bookmark the same page multiple times (maybe unrelated), I noticed that some already loaded tabs are converted to empty Speed Dials.
And Flash support is gone (no surprise here). That means no embedded online music players, embedded in page podcasts, some videos, etc. Will need to keep Opera 12.10 or Firefox for this.
Opera Link is lost? Or it's temporary solution to use bookmark to web page...
Double taping on images that zoom them to 100% is lost. Back button was useful to bring zoom back after this.
If you tapped on a links that are too tiny and close to each other Opera highlighted both links and automatically zoomed in, so you can confirm your selection.
Just noticed that new Opera Mobile is missing nifty icon animation when you open tab in background.
No more questions for download location (can't choose a folder) as it was previously.
BTW, something gone wrong with installation. It renders pages only in Off-road mode, in normal mode it always show blank page. And with other Wi-Fi spot at work new Opera Mobile started to render pages normally, but after I came back to home it again back to render blanks (tested this behaviour couple times already).