It looks like Ruby and Python

fibonacci = (x) ->
  return 0 if x == 0
  return 1 if x == 1
  return fibonacci(x-1) + fibonacci(x-2)

fibonacci 10  # returns 55
      

It's hard to deploy and debug

TypeError: Cannot call method 'updateAttributes' of null
    at Promise.Handler.handleUpdate (/myapp/lib/handlers/handler.coffee:55:22)
    at Promise.addBack (/myapp/node_modules/mongoose/lib/promise.js:128:8)
    at Promise.EventEmitter.emit (events.js:96:17)
    at Promise.emit (/myapp/node_modules/mongoose/lib/promise.js:66:38)
      

It gives me a bunch of great language features

congratulate(user) for user in @users where user.isWinner()  # loop comprehensions
"Hello, my name is #{@firstName} #{@lastName}."              # string interpolation
for i in [0..10]                                             # ranges
subset = myArray[2..5]                                       # range slicing
zip = account?.user?.address?.zip                            # existential operator
{ number, street, zip } = @getAddress()                      # destructuring
      

It doesn't support named functions

// Y u no possible in coffeescript?

function square(x) {
  return x * x;
}
      

It helps avoid typical JavaScript bad practices

var foo = 3;      // good
foo = 3;          // really bad

false == ' \t';   // this is true... wtf?
false === ' \t';  // much better
      

It makes a prototypal language look like classical inheritance

class User
  constructor: () ->
    @admin = false

  fullName: () ->
    "#{@firstName} #{@lastName}"
      

It's easy to write readable DSLs

require('zappa') ->
  @use 'bodyParser', 'methodOverride', @app.router, 'static'

  @configure
    development: => @use errorHandler: {dumpExceptions: on}
    production: => @use 'errorHandler'

  @get '/': -> @render 'index'

  @on connection: ->
    @emit welcome: {time: new Date()}

  @on shout: ->
    @broadcast shout: {@id, text: @data.text}
      

It has started Yet Another Religion War

function js(region) {
  if (region == null) {
    region = 'world';
  }

  return 'I am the king of the ' + region + '!';
}

cs = (bodyPart = 'ass') ->
  "Kiss my #{bodyPart}, grandpa!"