Thief classes have been implemented well in other games, especially Deus Ex and No One Lives Forever 2. Both games have freeform skills (like in RiftRunner. The only problem is when you come up against bosses. A thief needs a skill that can be very powerful against a single opponent. Their thief skills should be required to get past the army.
Come to think of it, it shouldn't be a Thief, but rather, an Assassin. Sneak in, kill, escape.
Not to forget Thief: The Dark Project and Thief II: The Metal Age (and Thief 3 in production).
Also games like Splinter Cell and a couple of scenes in Metal Gear Solid rely on stealth.
