Now a weakness of having viruses on your computer could be that the address that you want to send the coins to can be modified before your PC displays it. IE you receive a PM and the virus changes all bitcoin addresses in your webrowser to their address or if you copy/paste it changes the addresses in the clipboard. So then when you THINK you're sending the bitcoins to the right person, it could actually be the wrong address. That is the #1 thing I would watch for.
I'm sure the trezor team can give you better descriptions, but that's my 2 cents
This is the purpose of the screen, to verify the address and amount are correct before signing.
What I'm talking about is a virus giving you the wrong address to send to, which occurs prior to dealing with the Trezor at all. Like if I email you my address which is 1Lyth and the virus changes the HTML/message to read 1Virus and then you send the bitcoins to 1Virus thinking that it's the correct address.