Ctrl+Tab does not show a list
is it a bug or a lack of feature?
Your first post, so welcome!
I don't think it is implemented yet (neither are several other much requested shortcuts), but it would be nice! (…and undoubtedly will eventually be included)
In the mean time, Ctrl+1,2,3,4 can be used to cycle through tabs, but without a menu/list. If you're not already aware of them, you can see and edit the currently available shortcuts in the settings.
If I liked switching by tab number or by cycling through list, I would not search for an exotic unstable browser that claims to replace Opera 12. I would rather use firefox/chrome/whatever. Okay, will wait.
Is there a structured feature list anywhere? Not a 39-page topic, but a bugtracker or something like this.
Regarding a structure feature list or other documentation, no, Vivaldi is too new and all the developers' energy is going into getting usable product ready. Currently it's at the Tech Preview level (variously interpreted as alpha or pre-alpha; some projections mentioned in the forum have said maybe a beta level browser by the end of the year. If you haven't already seen it, check this Vivaldi home page for an overview of the project goals.
If you like tinkering with a browser and can tolerate a lot of bugs and unfinished features, Vivaldi can be a lot of fun. Eventually it should have as much or more customizability as Olde Opera (12.x and earlier, before they switched to Chromium/Blink). But depending on how you look at it, it's really in the gestation stage, or neonatal stage, or at most the early toddler stage right now.
If you do want to use it and are using Technical Preview 2 (v.184.108.40.206) as most new users are, I would strongly recommend the most recent Developer snapshot.
Vivaldi is developing rapidly and there have been numerous bug fixes and feature improvements since Technical Preview 2, and Developer snapshots since then have generally been as stable, if not more stable, than TP2.
Follow the Team Blog for weekly updates (usually on Mondays) and occasional mid-week updates. You could probably get a pretty good idea of where things are at by reading the Team Blog posts (usually with a list of bugfixes IIRC) over the last couple months. (Extra credit for wading through the Blog post user comments :P)