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
Thanks all for the great responses. You inspired me to research the heck out of this. I'm very excited to report that I completed my order for an Origin Live Sovereign turntable with the Origin Live Conquerer tonearm and the Dynavector XV-1s cartridge.

The Origin Live units have been getting great reviews (e.g., the latest Bound for Sound) and I was able to get a very fair price on this combo which is just slightly used. I'll be plugging that into my PBN Olympia fully balanced phono preamp (FET, separate battery power supply). I should be getting the whole rig in a couple of weeks and will report my findings.

Thanks again. I ended up spending more than I thought I would, but the price was right and this'll save me an upgrade later ... famous last words ;-)
Dear friend: You can't say " I want to buy a tonearm ". When you want to buy a tonearm, you always have to think about it " wife's ": the phono cartridge. You can't separate, tonearm/cartridge is a "combo buy ". Go with you audio dealer and talk about this issue.
Regards.
Raul.

If you're low on cash and have moderately skilled hands, buy a used (meaning cheap and old) Rega Planar 3, and rebuild it; plus, do all the tweeks, thereby turning it to the equivalent of the "Sota Saphire". You also get the tone arm with the TT, which can be upgraded with new wire.

All the instructions for doing this are on this forum.

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).