Meteor.js vs. MEAN stack
Disclaimer: This is my personal view on Express/MEAN and Meteor. I have used both for developing applications.
The MEAN stack
The MEAN stack is just MongoDB, Express, Angular and Node.js bundled together. The main problem with this stack is that Express.js is too lightweight and programmmer has to do a lot of things on his own. For example authentication and registration can take a few hours to implement in Express.js. You have to learn and understand Angular.js which is not so easy to learn.
What about building real-time applications? Handling sockets by hand is out of fashion but you have to do it in Express.
Meteor.js for the win
Meteor handles all the issues mentioned above and even more! Do you want to show your application to the world but you don't want to pay for hosting yet? Meteor provides free hosting for Meteor apps on meteor.com subdomain. You can deploy your application with one command and your application is on the internet:
meteor deploy yourappname
Meteor has a lot of packages built by its community. You can basically build your application just by putting packages together. Do you need authentication and registration? Just add a package for it. Do you want to use LESS or CoffeeScript? Just add packages for it...
Don't waste time with technologies that are difficult to learn and where you have to do a lot of work by yourself. Meteor is easy to learn. It's the easiest technology to learn and the most powerful. Just try to write an application with Meteor and you will see. You will be surprised how simple Meteor.js is.