Intuitive and attractive user interface. It may seem clichéd, but there's some truth to the statement "beautiful things work better."
It's been years now but Morten's question below has been stuck in my head all this time. Yes, I do judge SQL servers by their user interface. Everything has a user interface. Well, almost everything anyway. Shells, config files, commands, log files, they all are an interface for users. Even most linux kernel modules have some UI exposed as files in /sys and /proc. Yes, those are user interfaces. Just because it isn't graphical doesn't make it any less important.
Example: I know Postgres is an excellent database. I'm sad sometimes I don't use it more. I use MySQL more. Why? Part of it is that the shell commands are easier to use.
show databases; says what it does and does what it says. I looked that up once, the first time. Never did again. When I use psql, I can rarely remember. Is it
\dl or just
\l? No wait, isn't
\dl something else? Huh, do I really want
\dn? Shoot, ok, what's the command for help?
\help? No, that's a list of SQL commands. Where is the list of shell commands? Ah.
\? has the list. WHY is
\? different from
Yeah if I used it all the time I'd get used to it. That's not the point. The point is that well designed software shouldn't require "getting used to."
I know that seems like a silly example but it does have an effect.