The features that one person finds useful may not be the ones that someone else values.
Along with the ability to customise the browser, one of the things I valued about Opera was that it had a lot of unique little features. Not necessarily things that everyone used, or even things that I used every day, but features that could be a nice time saver under certain circumstances.
For example, being able to tile two tabs side by side, and even make one of them a “follower tab”, so that it opened any link clicked in the other tab. Probably not something that most users even realised they could do, but something that I’ve found really useful. That’s the kind of unusual feature, unique to Opera, that I can see being lost in this transition.