To me a good tig welding machine is one that has a good crisp low amp start that wont blow holes in thin metal when you start the arc.
A good tig welding machine must also have high freq start, decent duty cycle, and enough power to weld the occasional aluminum block my machinist friend just crashed his tool changer into.
In a nutshell, Its got to have AC and DC current, good low amp starts, and plenty of sack.
2 settings I like on AC are AC balance and AC frequency.
Most every newer tig welding machine has these 2 settings and I probably wouldn't buy a new machine that didn't have both.
Everything else, all the bells and whistles, are gravy...they are much appreciated , but not completely essential.