The Full List of Awesomeness *genuflect*
Multiple Accounts
Quickly switch between accounts as you would tags in Google Reader. Replying and retweeting automatically get sent from the account they belong to. Unless you want to send them from a different account. That's pretty easy too.
Threaded Conversations and Reply Reminders
If you are viewing a conversation for the first time all the replies are threaded below the original tweet. If some slacker replies to a tweet two days after it was posted the original tweet is shown below the reply (dimmed until you mouse over it) to remind you what they are replying to.
Search
Search all your old tweets or search for tweets from a particular user with "@username keyword".
No Syncing + Mark Items as Read
Since 20Couch is a web app you can access it from any computer or mobile device without having to install anything. Even better as you jump from your work computer to your iPhone to your home computer you don't have to worry about syncing or finding where you left off.
Mute
Say you're following @sportsguy33 and he starts tweeting about college basketball and you have no interest in college hoops you can simply mute him for an hour. Or is you are following the tag #jquery, but it's constantly spammed by @jeresig you can mute him forever and never see his spammy tweets again.
iPhone and Mobile Browser Ready (beta)
Visit your 20Couch in a mobile browser and the layout automatically switches to be a better fit. Unfortunately this feature is still in beta, since I don't have an actual smartphone to test it on.
Inline Pics
When a tweet contains a link to a twitpic.com or yfrog.com image a thumbnail is shown inline with the tweet. This is great for when you're catching up on your tweets at work and someone decides to post a NSFW image.
Follow Hashtags
Track hashtags (or any search word really) just as you would follow other users. It's a great way to keep up on topics you're interested in or watch for people talking about how great your Twitter client is.
Ninja Follow
Let's say you want to keep up with all the latest image from @fybg, but you really don't want to follow them through Twitter because then everyone would know. NINJA FOLLOW!! But wait. There's more. When you ninja follow someone you also get all their @replies. Hypothetically, say I'm super into Twilight and want to know everyone that @robTompattinson replies to so you can kindly visit them and tell them to stay away from @robTompattinson or you'll be forced to go all sparkly vampire one them. NINJA FOLLOW!!
One Click Update
Simply click "Setting"* in the top right, then on "Update"*. 20Couch will tell you if you're up to date. If you're behind, even more simply click "Auto Update" and you'll be updated to the latest version.
* click does not count towards one click total
Hide Retweets
Yeah, pretty much what the title says.
Short URL Expando
Mouse over a link and it'll show you where it's going without having to click. Only works for bit.ly at the moment.