Let's be real here, Twitter ruined Tweetdeck ever since it bought the company. [Old version of Tweetdeck here]. The new version of Tweetdeck is awful and is actually hugely watered down compared to what it was before.
Tweetdeck used to be the boss when it came to Twitter clients. It was the most used Twitter client out there, Twitter realised and knew they had to buy them out to compete with their very own app.
Tweetbot is now trying to be the next Tweetdeck, but a lot better looking and unfortunately limited to Apple's line of products.
The new version of Tweetbot for Mac ALPHA adds a feature which was a USP for Tweetdock, extra columns. In v0.6.3 of Tweetbot for Mac, Tapbots (the devs) added a feature which has been massively requested.
Columns give you the ability to look at more on Twitter at once. Instead of having to click on the "@" tab to look at your mentions, you can have them right there. However Tweetbot hasn't limited this to just a few columns, you can have columns for anything. Including lists, other accounts, mentions, searches, profiles...etc.
To make a new column just head to whatever you'd like to make a column for, like your mentions, then hit the little gear icon you see at the bottom right and you'll be presented with "Open in new window". However if you want to keep all these windows together like I have in the screenshot above, you drag the window close to the edge of the main Tweetbot window, it'll then snap into place. To detach a window, you simply click on the bottom of the window and then drag away.
You now get a little taskbar icon for Tweetbot. From which you can look at your different profiles, quickly access tweets, mentions or messages. Not to forget, notifications have also been added and they work perfectly with Mountain Lion.
Tweetbot for Mac seems nearly finished now. I think all they're going to do now is add a few more little features and iron out the UI and animations to make them silky smooth.