YubiKeys are JUST for securing an online account. A Trezor (or Bitcoin client) could act as an identity in of itself! It's not 2-factor authentication but a single source of authentication that can be identified and tied to a Bitcoin public key.
To clarify:
Pretend my identity is: 153eqRZVdUXp1LbKD3geJWMYL5NSmwYWMr
I can use the Trezor to guarantee no one else can fake that identity. To sign in with that identity I just sign random data that website gives me to sign in. No one else, unless they have the Trezor, can sign the random data with the Bitcoin address 153eqRZVdUXp1LbKD3geJWMYL5NSmwYWMr.
I could even post a message:
"I am dillpicklechips and I approve of this message"
IIqGxd1wDMhhab7OF7nqo3NZXiFOC8iLcnc0/rGQ57q9s3YA/4ecpUFc27SRJRhO9OP2IDH2vuiY4UjPd/TSL5k= (made up signature would be something like this, not valid though)
Only someone with my Trezor could create a valid signature. It allows a lot of cool uses where no one can pretend to be you without the actual hardware!
Passwords, 2-factor auth, could all be a thing in the past! The Trezor keeps the private key hidden and secure. As long as I keep the device safe, my identity is safe. And for the online world that is a VERY exciting thing!