What is your preference...
1. Costumed appearance (Superman, Wonder Woman, etc), or noncostumed (eg, Silver Surfer, Sasquatch)?
I like a good costume, though I really despise capes 😕
2. Elaborate appearance (eg, RK Thor), or simple (Daredevil)?
Elaborate usually looks cool, but it also stands out. It's kinda distracting and campy. But sometimes it works really well.
3. For superpower source: radiation, mutation, alien, divine or tech?
Well, radiation is a big nono these days. I don't really care what it is, as long as its relatively plausible within its own context. I also like power sources to be mysterious and multi-interpretable.
4. Closed powerset (eg, Superman, Flash), or open powerset (eg, Silver Surfer, Dr. Strange)?
In general, closed powerset. Easily. Unless we're talking higher beings with a god-like status. It wouldn't be right to limit their powers. But I do believe that a closed powerset helps most lower tier characters to develop more naturally. Part of the fun for me is learning how a character deals with his or her powers and their limitations.
5. Power level: low, mid or high tier?
Doesn't matter at all, as long as the character is good.
6. Conventional moniker (eg, Spider-Man, Batman), or more unconventional (eg, Hulk, Dr. Manhattan)?
Doesn't matter at all, as long as the character is good.