The improvements they are making to IPB look really great. Some of the highlights for me:

http://community.invisionpower.com/blog/1174/entry-9537-ips-40-editor-part-1-content/

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Currently URLs which gets picked up for media embedding and the output they produce are managed in the Admin CP. The idea was that this would allow admins to add support for additional services, however, it has caused some problems when for example, YouTube has added new URL formats or changed their embed code and they don't work on most communities until we release an update to change the default record for YouTube.

oEmbed is an API which allows you to make a call to the provider giving them the URL and they respond with the best embed code to use, if that URL can be embedded. In 4.0, if a URL for YouTube, Flickr, Vimeo, College Humor or Hulu is inserted, we make a call to their oEmbed provider to obtain the embed code, so it's always up to date.

Embeds will always work. Now that is a huge difference.

http://community.invisionpower.com/blog/1174/entry-9538-ips-40-editor-part-2-uploads/

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The upload panel here is based on HTML5 which supports drag and drop uploading, if your browser doesn't support this, it will use Flash, Silverlight or Google Gears if you have any of those installed, and if not it will fallback to a HTML4 & JavaScript implementation (none of these support drag and drop, but instead you click the "Choose Files" button just as you do now - the label in the box will change to reflect this).

Drag and drop images right into the editor. I've been waiting for this feature. No more having to click attach and then navigate to the right folder where your image is.. just drag it right into the editor.

http://community.invisionpower.com/blog/1174/entry-9539-ips-40-editor-part-3-customisation-and-bbcode/

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BBCode

Though no features in IPS4 insert BBCode-style tags into the editor (like is currently done for attachments, etc.) users can still type BBCode into the editor and it will work fine.

We've rewritten how BBCode is parsed to be much more secure and reliable and produce more standards-compliant HTML (for those who are interested, it parses the post content into a DOM Document and examines only the text nodes for BBCode tags, then either splits the nodes surrounding it and inserts one for block-level elements, or wraps all subsequent text nodes in the appropriate formatting element until the end BBCode is found).

The benefit to this is that there now no longer needs to be a "BBCode mode" - you can type BBCode straight into the editor, even complicated stuff like lists spanning multiple lines, and it comes out looking great.

The downside to this approach is that custom BBCodes can no longer be added through the Admin CP. However, as mentioned above, we now have the ability to add custom buttons to the editor which work in a much more intuitive way, and can do everything that custom BBCodes could and more. For those who really want to be able to add the ability for custom BBCode, we've isolated the method that returns the supported BBCode (and information needed to parse them) into a specific method so that custom BBCode can be added with a very simple hook specific to that purpose.

This is a big change and I hope they pull it off correctly. All our custom bbcodes will be negated. But the only custom bbcode we actually use is the spoiler tag, which will now be a default feature. All other custom bbcodes will now have to be created as CKEditor plugins.. but there are already a lot of ckeditor plugins available. So I could technically add a variety of new editor features.

http://community.invisionpower.com/topic/388306-ips-40-editor-part-4-special-features/

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@mentions

@mentions are a common feature on social media sites like Twitter and Facebook. If you type an @ symbol and then start typing the name of a friend, an autocomplete menu shows so you can quickly then click on the user and they'll receive a notification that they've been mentioned. In 4.0 you can do exactly this to mention any user.

Very very nice. I like this.

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Automatic Saving

Currently, when you're typing a post, every 2 minutes the content of the post is saved, so that if you accidentally navigate away from the page, your post content can be recovered. The content is saved by making an AJAX request.

In 4.0, we've rewritten this to use HTML5 web storage. This unloads this work to the browser, meaning no call needs to be made to the server. Because this is much more efficient, the save can be done much more frequently (every few seconds). This makes the autosave feature much more useful.

In addition, we've expanded the feature to support attachments. So if you've uploaded files, these too will be automatically recovered. Essentially if you're in the middle of typing a post and you refresh the page, everything will reappear exactly as you left it.

Good! No more having to click the load saved content option. Simplifying this will make the auto save much better.

Oh! and the editor will now be fully skin-able! The editor won't have to be glaring white on a dark theme anymore.

Also, the current skins that we use are going to be upgraded for IPB 4.0. We won't have to change the entire look of the site.