why the options are not available in the new versions.

Because none of the code from the Presto based Opera could be migrated over to the Blink based version. They basically had to start from scratch.

  I hope I can just left-click a bookmark on the bookmark bar to open it in a new tab. 

Thats working fine for me. I can right click links in the bookmarks bar and hit open in new tab just like anywhere else.
As for the loss of features, as has been stated many, many, many times the new Opera is a completely different program from the original. They are completely rebuilding it from the ground up and some features haven't been recreated yet.

Thanks again. In that case, I'm going to use an old version of Opera instead. On this page http://www.opera.com/download/guide/?os=windows&list=all
there are quite a few old versions that can be downloaded. Which is the latest of the old versions that include those features. Is it 12.17? The difference in value between this version and its next is the biggest.

I've found out that version 12 is really the latest that provides lots of options and configurability, many of which are just what I've been looking for, and why the options are not available in the new versions.

Thank you, everyone.

I hope I can just left-click a bookmark on the bookmark bar to open it in a new tab.

Thats working fine for me. I can right click links in the bookmarks bar and hit open in new tab just like anywhere else.
As for the loss of features, as has been stated many, many, many times the new Opera is a completely different program from the original. They are completely rebuilding it from the ground up and some features haven't been recreated yet.

Thanks again. In that case, I'm going to use an old version of Opera instead. On this page http://www.opera.com/download/guide/?os=windows&list=all
there are quite a few old versions that can be downloaded. Which is the latest of the old versions that include those features. Is it 12.17? The difference in value between this version and its next is the biggest.

(I do not like all these right-click and ctrl+click solutions anyway. Maybe you will start some topic in suggestion box, about opening bookmarks in 'sensible' way?)

I hope I can just left-click a bookmark on the bookmark bar to open it in a new tab.

Thats working fine for me. I can right click links in the bookmarks bar and hit open in new tab just like anywhere else.

As for the loss of features, as has been stated many, many, many times the new Opera is a completely different program from the original. They are completely rebuilding it from the ground up and some features haven't been recreated yet.

That help page is from a old version of Opera and doesn't apply to the current release. In the current version you right click the bookmark and click "Open in a new tab" or you can hold down the Ctrl key and left click the bookmark. This is from the bookmarks sub-menu in the main Opera menu. If you are on the bookmarks page inside Opera you just left click the link and its default action is to open in a new tab.

Thank you very much, lando. I hope I can just left-click a bookmark on the bookmark bar to open it in a new tab. And I don't know why the designers of Opera choose not to allow users to change some settings which could be modified by users in old versions. I also learned from a help page that dragging a link down would open it in a new tab, which is not available in my Opera 26.0 either. : (

It seem that I'll have to install and run Google Chrome to see if it is more convenient to use. I used to use Maxthon due to its perfect mouse gestures and features that could be configured. But it is not very robust, so I've decided to look for another suitable browser.