I have inherited an old Pioneer PL-518 Direct Drive turntable.

Since I haven't really heard any other turntables, I'm curious about the quality of this one. Is there anyone who has heard anything about this particular model? Is there anything I should look for when playing records (other than inconsistent speeds and other obvious things) that would be a good indicator to me regarding the quality of the turntable?

If you like the sound of it then that's all it matters but in gereral, Audiophile turntables are belt driven. Why? because the belt adsorb the vibration generated by the motor. Just look at a Gyro Orb and you'll see all the funny shape have their reasons. Isolation are also important, just like you want to put your CD player on a stable platform, it applies more on a turntable.

Listen carefully in between tracks or during very quiet passages. Do you hear a rumble from the motor? Belt driven systems will reduce this significantly. Also listen for any grinding/slurring/wheezing noises which will improve by going with a better table and/or arm.

I've just ordered yesterday a pretty good turntable myself that will replace my twenty year old marantz and when I auditioned it at the dealer the difference is so huge. Close your eyes and you think you can reach out your hand and touch the instruments, it's that good.