* Creating a channel is weird from a UX perspective. You hit the plus arrow and a text field comes up, but there isn't any indication of how to submit it. Hitting Enter works, but not having a button is a weird UX choice.
* The plus button works as a visibility toggle but doesn't change from + to -. I would add a + button next to the Channels title that opens a mini form with a text field plus Create and Cancel buttons. You can have the Esc key close the dialog, but there should be clear buttons showing what does what.
* What is the purpose of the left-hand tray? Is it for eventually having access to multiple servers, or is it just a logo? It takes up real estate without providing a use. If the server feature isn't going to exist for a while, it should be removed and the logo placed elsewhere.
* The user's avatar is very large with some properties around it. This should probably follow the more common convention of being an icon in the top right, with the settings in a properties panel or page. Once set, the user isn't going to interact with the profile that much, so more room should be given to common operations.
* There isn't really a theme, or at least the look is very sparse. Setting just a color is fine, but it would be nice to have the ability to set a swatch or theme of different colors, like an IDE theme.
* Contrast is harsh for different colors in the theme. Colors for buttons, labels, and icons should be calculated to have adequate contrast with the primary color