I'm trying to find something more advanced then Chrome, so I tried to use Opera for some time. Here is result of my experiment. I'm using Opera 46.0.2573.0 on Windows 10.
#Most annoying things
Clicking Opera taskbar icon opens new tab. It should open new window.
'Go to web address' page context menu opens URL is current tab. It should open it in new tab.
Tabs are closed by mousedown of middleclick. They should be closed by click.
All tab favicons in 'Private window' are replaced by one strange icon. Because of this, tabs become indistinguishable and using 'Private window' becomes a mess.
After installation, Opera silently imports bookmarks(and maybe some other things) from default browser. This is totally unacceptable. You may only show 'Import bookmarks and settings' window soon after installation. But silent importing looks disrespectful of user.
Option to not show close tab buttons. They only waste space and sometimes when I want to select another tab I occasionally click on this X button and tab is closed. Using middleclick is enough.
Duplicate current tab by middle clicking Reload button.
Tabs like Extensions, History and Downloads does not display URL in address bar. Also I can't add them to bookmarks bar by drag and drop. But I can open Console, run 'location.href' and see tab URL. I can copy it, then start editing existing bookmark and paste URL in Address field and I've got working bookmark. So URLs like 'chrome://startpage/bookmarks' should be displayed and it should be easy to add such tabs to bookmarks bar by drag and drop. And the same situation is with pages inside extensions(for example Options page): URL is not shown. I can't copy it without opening Console.
Popup from 'Tab menu' button with 'Recently Closed' and 'Open Tabs' is very handy. But why it is so narrow? It should be at least twice as wide. Modern widescreen monitors have enough space for it.
'Search Google for image' context menu for images.
'Remove' and 'Disable' items in context menu of extension icon.
Sorting extension icons with drag and drop.
Selecting several tabs with Ctrl+click.
Ability to place extensions sidebar not only in the left, but also in the right and maybe in top or in bottom.
Now there is an interesting possibility to open and dock Facebook Messenger, WhatsApp and Telegram in main(configured by opera://settings/sidebar) sidebar. But why only this sites? Maybe add possibility to place icon and dock any URL in sidebar? It can be YouTube or some music playing site.
Another idea: place icons of sidebar extensions in main sidebar and so extensions sidebar(Ctrl+Shift+S) is not needed anymore and some space is saved.
Middle clicking icons like History, Extensions, etc. in main sidebar opens them in new tab.
Middle clicking 'Press to open a new tab' button opens new window.
This method works good and returns array of Session objects. Session object has 'tab' property of Tab type. This object property should have 'favIconUrl' property, but it is totally absent in Opera, even if extension has 'tabs' permission.
Sometimes I create new tab, paste URL in it, press Enter and this new tab is moved to new window.
#Things I liked in Opera
Overall UI and tabs appearance.
Separating extension icons to 'page action' and 'browser action'.
'Tab menu' popup with 'Recently Closed' and 'Open Tabs' sections.
Sidebar API for extensions.
Search field in Extension tab. Ability to group extensions by Disabled, Under development, etc.
Showing download speed and grouping downloads by PDF, Images, etc.
'Hide from toolbar' option for extensions.
Absence of startup warning, when using extensions not from Store.