What makes a good tonearm?


I'm entering the world of decent analog and believe I understand why the turntable design is critical (e.g., vibration isolation, balance and speed stability) but am scratching my head a little about tonearms. What makes a good tonearm good and a great tonearm great? The resulting sound is always the key, but what design characteristics contribute to that sound? Tonearms are a big investment and can be tough to change later so I'd like to understand more about them before I buy anything. Thanks in advance.
ozfly
Check out the Audiomods arm. Made in the UK by a master machinist, who uses only the Rega 303 arm tube, all other parts made by himself. A very interesting set of design choices.
Another thread from 2004 is up now. C'mon, the price for Audiomods tonearm is higher than some absolute favorite vintage arms like Technics EPA-100, Victor UA-7045, Lustre GST-801, Sony PUA-7, Luxman TA-1 and many others including the brand new Schick "12, Jelco etc. 

The new arms are not designed for high compliance cartridges and this is the problem. What make a good tonearm goog is the synergy with your particular cartridge. 

Some of the old tonearms are underrated lon the used market, like the Victor UA-7045 - this is absolutely the best buy for those who would like to save money without loss in performance level. Amazing tonearm (imo).  
I get great results with Jelco SA-750 (9" and 12"). They run Decca/London cartridges beautifully. If they can do that, they can handle anything! (except high compliance cartridges)

chakster, I went from a Rega RB300 to a  Victor UA-7045 and loved it. It's a very good arm for the money. Currently using a Pole star arm which mates well with my clearaudio MC cartridge.