Real questions have answers, not ideas, items or opinions.
Easy to say, but hard to always put into practice as you've noted. A simple "better than" encourages speculation. We don't want that. A good answer will come from the personal experiences of the answerer. Ideally, reading it we should know that they have been there and done that.
My suggestion when faced with a "better than" question is to ask the questioner, "better for what purpose?" and "Why do you want to know?"
A fleshed out question that I would consider valid is:
I'm running a variant of deck X and I'm having trouble with deck Y usually I get (weenie rushed, milled, whatever).
What strategies should I use against this deck? Are there cards I could add to my sideboard?
It's really the same question, just with solid details. These details should encourage people with similar experiences to give great answers. We want these types of answers on the site.
Practical answerable questions based on actual problems
We want to connect experts with people with actual problems, thats why I mentioned "Why do you want to know?" above.
Consider a simple question:
"What is the best blue flier?"
I could see this question coming from a new member of the site. They don't know any better, it's not their fault that StackExchange sites are not really forums. They look similar and the new person wants to fit in, participate and get a discussion started.
Our job is to politely query them to try to determine which of the below two situations the questioner identifies most closely with.
1 - Your buddy, who doesn't play much MtG but loves to talk about it. Recently he was on his couch thinking, "Blue is the best color, so many cool fliers. I wonder what the best one is?"
This person doesn't have a real problem. He just wants to talk. Let's sent him on to a forum.
2 - The serious player. She's been trying out a new deck for a couple weeks, took it to her local store. She did OK, but got crushed by certain decks. She thinks a blue flier might help.
This person has a real problem. Let's help her!