automatic CSS finding. I see this is already done with
This is already scheduled for the final release among other features you will not find anywhere else. As it stands, W3TC is already planned through v1.1 at which point I doubt there's more than I can transparently automate for the needs of _all_ blogs.
combining this plugin with free CDN ?
I'm not sure I see the benefit. At this moment only Amazon Web Services are not supported in W3TC (coming in v1.0). With W3TC, a user can use their own subdomain (via FTP) to improve pipelining and progressive render of their site or they can use a CDN that supports Mirroring (origin pull). W3TC already supports custom files, theme files, media library (also imports files into the media library) and wp-includes files with your CDN provider. That's everything. :)
I would love to know what features w3-total-cache gives that the two other major caching plugins don't (e.g: wp-super-cache and web-optimizer), and vise versa. To: frederick, I imagine you won't do it yourself, but know that I personally find it of interest. And in that I believe I reflect other people in the community as well.
Believe it or not, this question is not frequently asked once people review all of the options and the plugin's FAQ tab.
You also may not be aware, but there are *several* more caching plugins for WordPress available than those you listed and none to date combines server-side performance optimizations with client-side optimizations in a unified manner, nor implements practices that consistently benefit any WordPress installation (rather than a few specific use cases).
Anyway, the features and benefits on the plugin description are all things that no other plugin is reliably delivering at the moment:
WP Super Cache (WPSC) is only a disk bound page cache, that functionality is 1/4 of the W3TC suite. I've personally worked on blogs that saw no benefit from WPSC without the introduction of an opcode cache (like APC), so until W3TC there was no one-size-fits-all optimization plugin.
WPSC modifies WordPress' output, while W3TC performs optimizations of WordPress' output and caches those, thereby not interfering with plugins and functionality or requiring theme changes.
Among other differences between WPSC and W3TC: W3TC can cache pages on disk or in memory. Minifies CSS, HTML, JS (automatically downloads 3rd party scripts) and RSS feeds (in memory or on disk), integrates your site with a CDN transparently, handles cache headers, handles http compression, caches database queries (in memory only for now, but could offer disk in the future) and transparently supports multi-server hosting scenarios with ease all while making it available to shared hosting users. And it does it all without asking the user to modify .htaccess, their theme or how they work with WordPress. Also a user can create groups of CSS and JS per template in their theme, embed JS in a non-blocking manner in the head or footer of their pages etc because as you know some plugins introduce CSS + JS into post pages only, so now you can deal with that case.
Web Optimizer (WO) is client side oriented, only offering a couple features that W3TC does not yet have in a publicly available release, however it's also missing a few features that I would offer in a client-side optimization plugin. WO offers a number of features that have diminishing returns in practice or that cannot be fully automated reliably (requiring user input or theme modifications). Furthermore, some of the techniques available in the plugin require more automation than the plugin currently offers, which means that the operations would have to be run more than once, while W3TC on the other hand is fully automated and far more transparent. Yes, W3TC and WPSC are missing set up wizards, but that's something already planned for the full release, W3TC is at present only in beta. And again, all of the other differences I listed above are also applicable. W3TC promises something much different to the blog owner.
In essence, other caching plugins are doing a subset of what W3TC offers. This is the only plugin that can scale your servers and improve your user's experience at once with a set up that takes only a couple minutes. If you have advanced needs, W3TC steps up to meet those as well.
Most importantly this plugin is in play and bringing all of this functionality to major blogs on the web, which simply has not been done before. Savvy blog owners have arrived at some great results on their own marrying plugins together and doing their own coding, but that is not "duplicate-able." W3TC makes performance optimization "duplicate-able."
Web hosts are now able to increase their server density and be "greener" to the environment when the WordPress blogs they host use W3TC (making the blogs less resource intensive). No longer is user experience sacrificed for server density (packing as many domains on a server as possible without caring for the performance consequences).
To finish answer your question fully I'd have to repeat everything that's already in the FAQ in the plugin or the plugin description page. Please take a look at those in more detail.
When I enter CSS into minify. Then turn off the plugin, and then turns it back on - the CSS I entered will disappear (it might be worth keeping them stored someplace).
I don't think I will change this behavior because if you deactivate the plugin, to me that means that you don't wish to run it and as a result it should do housekeeping, which it does (permissions allowed). You can instead disable any functionality and keep your cache and settings, which is expected behavior as well.