Add Site Setting allow_uncategorized_topics. Uncheck it to force people to choose a category for all new topics.

app/assets/javascripts/discourse/controllers/composer_controller.js

@@ -42,6 +42,7 @@ Discourse.ComposerController = Discourse.Controller.extend({
 
     if( composer.get('cantSubmitPost') ) {
       this.set('view.showTitleTip', true);
+      this.set('view.showCategoryTip', true);
       this.set('view.showReplyTip', true);
       return;
     }
@@ -336,6 +337,7 @@ Discourse.ComposerController = Discourse.Controller.extend({
     this.set('content', null);
     this.set('view.content', null);
     this.set('view.showTitleTip', false);
+    this.set('view.showCategoryTip', false);
     this.set('view.showReplyTip', false);
   },
 

app/assets/javascripts/discourse/models/composer.js

@@ -172,8 +172,10 @@ Discourse.Composer = Discourse.Model.extend({
     // reply is always required
     if (this.get('replyLength') < Discourse.SiteSettings.min_post_length) return true;
 
+    if (!Discourse.SiteSettings.allow_uncategorized_topics && !this.get('categoryName')) return true;
+
     return false;
-  }.property('loading', 'editTitle', 'titleLength', 'targetUsernames', 'replyLength'),
+  }.property('loading', 'editTitle', 'titleLength', 'targetUsernames', 'replyLength', 'categoryName'),
 
   // The text for the save button
   saveText: function() {

app/assets/javascripts/discourse/templates/composer.js.handlebars

@@ -39,7 +39,10 @@
             </div>
 
             {{#unless content.creatingPrivateMessage}}
-              {{view Discourse.ComboboxViewCategory valueAttribute="name" contentBinding="categories" valueBinding="content.categoryName"}}
+              <div class="category-input">
+                {{view Discourse.ComboboxViewCategory valueAttribute="name" contentBinding="categories" valueBinding="content.categoryName" showUncategorized="true"}}
+                {{popupInputTip validation=view.categoryValidation show=view.showCategoryTip}}
+              </div>
               {{#if content.archetype.hasOptions}}
                 <button class='btn' {{action showOptions}}>{{i18n topic.options}}</button>
               {{/if}}

app/assets/javascripts/discourse/views/combobox_view_category.js

@@ -7,12 +7,15 @@
   @module Discourse
 **/
 Discourse.ComboboxViewCategory = Discourse.ComboboxView.extend({
-  none: 'category.none',
   classNames: ['combobox category-combobox'],
   overrideWidths: true,
   dataAttributes: ['name', 'color', 'text_color', 'description', 'topic_count'],
   valueBinding: Ember.Binding.oneWay('source'),
 
+  none: function() {
+    if (Discourse.SiteSettings.allow_uncategorized_topics || this.get('showUncategorized')) return 'category.none';
+  }.property('showUncategorized'),
+
   template: function(text, templateData) {
     if (!templateData.color) return text;
     var result = "<span class='badge-category' style='background-color: #" + templateData.color + '; color: #' +

app/assets/javascripts/discourse/views/composer_view.js

@@ -384,6 +384,12 @@ Discourse.ComposerView = Discourse.View.extend({
     }
   }.property('content.title'),
 
+  categoryValidation: function() {
+    if( !Discourse.SiteSettings.allow_uncategorized_topics && !this.get('content.categoryName')) {
+      return Discourse.InputValidation.create({ failed: true, reason: Em.String.i18n('composer.error.category_missing') });
+    }
+  }.property('content.categoryName'),
+
   replyValidation: function() {
     var reply = this.get('content.reply'), reason;
     if( !reply || reply.length < 1 ){

app/assets/stylesheets/application/compose.css.scss

@@ -322,14 +322,19 @@
       bottom: 8px;
     }
   }
-  .title-input {
+  .title-input, .category-input {
     position: relative;
     display: inline;
-    .popup-tip {
+  }
-      width: 300px;
+  .title-input .popup-tip {
-      left: -8px;
+    width: 300px;
-      margin-top: 8px;
+    left: -8px;
-    }
+    margin-top: 8px;
+  }
+  .category-input .popup-tip {
+    width: 240px;
+    left: 432px;
+    top: -7px;
   }
 }
 

app/models/site_setting.rb

@@ -35,6 +35,7 @@ class SiteSetting < ActiveRecord::Base
   client_setting(:max_post_length, 16000)
   client_setting(:min_topic_title_length, 15)
   client_setting(:max_topic_title_length, 255)
+  client_setting(:allow_uncategorized_topics, true)
   client_setting(:min_search_term_length, 3)
   client_setting(:flush_timings_secs, 5)
   client_setting(:supress_reply_directly_below, true)

config/locales/client.en.yml

@@ -343,6 +343,7 @@ en:
         title_length: "Title needs between {{min}} and {{max}} characters."
         post_missing: "Post can't be empty."
         post_length: "Post must be at least {{min}} characters long."
+        category_missing: "You must choose a category."
 
       save_edit: "Save Edit"
       reply_original: "Reply on Original Topic"

config/locales/server.en.yml

@@ -417,6 +417,7 @@ en:
     min_topic_title_length: "Minimum topic title length in characters"
     max_topic_title_length: "Maximum topic title length in characters"
     min_search_term_length: "Minimum search term length in characters"
+    allow_uncategorized_topics: "Allow topics to be created without a category"
     allow_duplicate_topic_titles: "Allow topics with identical titles"
     unique_posts_mins: "How many minutes before a user can make a post with the same content again"
     enforce_global_nicknames: "Enforce global nickname uniqueness (WARNING: only change this during initial setup)"