Meteor is my first nodejs framework and I have no experience with other nodejs frameworks. Meteor is full stack and has the most stars at github if I'm not mistaken. It seems Meteor can everything that other frameworks can and much more with atmosphere and npm extensions. Why/when should I choose other frameworks? I read several sites with overview about top nodejs frameworks, but they don't answer my question.
Meteor is quite versatile, but is aimed toward mobile app development first. It is all good and dandy, and you may not see any need to go outside of Meteor.
But what happen the day you have to do a real quick HTTP server?
Express will beat it because it is it's home turf.
Want to do real time?
Socket.io will wreck it.
You may Meteor will be "good enought" for thoses task, and other I couldn't think of the top of my head, but one day or another, you will either encounter a problem for which Meteor is not suited, or have a requirement forbidding Meteor.
That day, you will want to have already learned one or two other framework, just so you don't have to panic and learn another without knowing if it is a good fit, and making something half baked because you were learning on-the-fly.
TL:DR: Node.js frameworks are like a tool box, you may be fine in the foreseable future with a screwdriver, but the day you really need a hammer, you don't want to hit yourself on the fingers. Better to also know how to handle the hammer before you have to put a nail.