A couple of things:
Hansam, you need to understand even in standby the phone is still doing things in the background, so it's still going to be using power. Whether you touch it or not. The radios are on so that when you get a message the phone can receive it. The OS is running all the time to serve any application or hardware requests. Memory has to be refreshed. It's a computer, and if it's on there are things that will use power.
Since you didn't start with a fully charged phone, it's hard to know what your battery life really is. How long did it take for the battery to drop from nearly full to 50%? Next time, make sure you charge the phone to 100%, which will reset the battery monitor, and then track usage until you're down to about 50%, and repost the images then. But IF you're really getting 8 hours of battery life at 50%, that's really nothing to complain about. If you didn't charge to 100%, that's probably not accurate, though. To repeat: You MUST charge to 100% for the battery stats to really be meaningful.
Media gets used any time you download any file. So if you downloaded videos, or programs, or any large file that will account for quite a bit of usage. And media server gets used whenever you watch a video, whether you're watching it online or not. So if you downloaded those videos, and then watched them offline, that could account for quite a bit of battery.
But since your phone isn't going to sleep, something else is probably wrong. Heavy media server usage can be caused by corrupted media files. Are your videos on a removable SD card? If so, try removing the SD card, reboot, and see if that fixes the problem. If they're on internal memory, I'd suggest copying any files you want to keep to your computer, delete everything from the internal SD, reboot and see what happens.
So: remove the SD card if that's where the files are. Delete the media files from you internal SD if that's where they are. Reboot the phone. Charge to 100%. Let us know what happens.