Bricked my Asus TF300T? Manual JB update
Solved. ASUS released the new firmware in its website.

Flashing it unbricked my device:


Download the stock firmware from ASUS’s official website. On the website, select Android in the OS drop down menu, then select Firmware, and download the latest firmware.
Extract the downloaded firmware file. You will get another archive file. Extract this one as well to get several files. One of these files will be named “blob”, which is the one we need to use.
Install Android SDK for Windows.
Shut down the tablet. Hold Power + Vol Down buttons until a message appears saying “Press Vol. Up to enter RCK (5s)”. However, DO NOT press the Volume Up button. Instead, wait for 5 seconds till a new menu appears. Then, in less than 10 seconds, press Volume Down once to highlight the USB icon, then press the Volume Up button to validate.
Open Command Prompt on your computer from Start » All Programs » Accessories (or open Start menu, type cmd and press the Enter button).
Now, type the following commands in command prompt to flash the stock ROM, pressing Enter after each command:

> cd C:...\Path-to-Andoid-SDK\platform-tools\

> fastboot devices

> fastboot -i 0x0B05 flash system C:...\Path-to-blob\blob

> fastboot -i 0x0B05 reboot


Source: http://www.theandroidsoul.com/how-to-restore-asus-transformer-tf300t-back-to-stock/

More details: http://forum.xda-developers.com/showthread.php?t=1846661