The threshold voltage is the voltage at which the device turns off, not on. More specifically it is usually quoted at a few 100 _micro_amps. Ignore Vthr completely for switching applications, the way it works is this:

If Vgs = 0V the device is off.

If Vgs > Vgs(on), then it is fully on, where Vgs(on) is the value quoted in the datasheet under the Rds(on) entry.

The datasheet tells you the on-resistance for a given Vgs - any voltage less than that is not guaranteed to turn it as fully on (or at all).

Logic-level MOSFETs will have something like "Rds(on) = ... (Vgs=4.5V)". This means the on-resistance is guaranteed for logic level drive.

Non-logic level MOSFETs only have Rds(on) quoted for Vgs=10V.

Sometimes values for Rds(on) are given for both 4.5V and 10V, this is a logic-level device basically.

Typical values for Vthr are much less than Vgs(on), about 1/4 of it in fact. There is another voltage, the plateau voltage, at which most of the current switching happens - this is somewhere between Vthr and Vgs(on). Datasheets have a graph of typical gate charge v. gate voltage - have a look at it and see the plateau. If you provide a Vgs thats too low you may fall below the plateau voltage and the device will conduct only a small fraction of its rated current.

You only ever use common-source circuit configuration for switching in practice.