It depends on what you need to do with it. All these factors are trade-offs.
Structural rigidity depends on the diameter of the leg tubing,  for the most
part. Larger, thicker tubing is heavier and costs more. Materials such as
carbon fiber and Gitzo "basalt" are about 1/3 lighter than aluminum and
about equal in rigidity, but cost significantly more. So here you balance
price against weight. If you do a lot of trekking you'll probably want to
spend the extra money; if you do mostly setup shots in the studio it would
hardly be worth it.

Generally I find that it is nice to have a set of sticks that will be at my
eye level without raising the center post. I also have a little Gitzo that
has a range from about 35cm to 1m for just packing along easily. Most of the
photojournalists I know have big heavy beasts that go up to about 2m. With a
200mm lens you don't need anything too heavy-duty, but keep in mind that no
tripod is completely steady once you introduce motion into the picture--even
the most heavy-duty ones will vibrate to some degree if the wind picks up,
or due to camera mirror-slap.

Gitzo and Benbo make unique tripods in which the legs and head post can be
set at any angle, as they are all aligned on a common axis. They are
excellent for working on mountainous or angled terrain, or setting up macro
shots in the field, but they are not as easy to use as a standard tripod.
It's all trade-offs, and what you need.

Personally I like a ball head, since I can adjust all the movements with a
single knob, but others prefer three-axis heads.

Another question is about having a geared center post. Non-geared is lighter
and generally faster, but geared allows for very fine adjustments, which can
be very useful in macro work.

Do you have more specific questions?

Toby

It depends on *greatly* on what you want to do with a tripod.
I've got several (most of them relatively inexpensive too), and
commonly use at least three of them.

In addition to your list, there are some other things which
might (or not) be important.

The type of mount for a head (for example if you have more than
one tripod or more than one head, being able to interchange at
least some things is useful).

Another possibly useful consideration is what type of accessories
are available for it.  Sidearms for example, to mount the camera
off center from the tripod.

The ability to get the camera down low for taking pictures of
things at ground level (flowers, bugs, whatever).  And perhaps
the ability to have the camera at a low level to match your
height when sitting on the ground.  (That's not one I do, but
I've seen sports photographers on the sidelines at football or
soccer games sitting on the ground with a tripod that has legs
spread way apart, and the camera right at eye level.)

Obviously one tripod cannot be all things to all situations.  I
have a very lightweight tripod with a ball head that I use when
I have to carry things, but don't need super stability.  Setting
up for shooting social functions indoors, for example (games,
political rallies, school functions, etc. etc.).  On the other
hand I have three tripods with Majestic gearheads on them, and I
just love them all.  The tripod I use (and love) the most is my
one genuine Majestic tripod (made by Bencher).  I have two
different kinds of sidearms for it and it gets used for just
about anything where I can tolerate packing a tripod that heavy.

weight and rigidity seem to be related, dead weight counts quite a bit, (but
its easy to add weight with a plastic bag and some rocks, water bottles
filled on site and tied together and looped over the cross brace.

the cross brace helps a lot.  it makes the pod a solid form and not 3 sticks
in a hub.  triangles are the strongest structure.   in the old days they
used a triangle on the ground and put the spikes into the ends.

ease of use, quick release clamps.  cheap ones cause loose joints that can
let the camera vibrate, good ones can cost as much as the pod.   pneumatic
legs lets you set up faster, makes adjusting a whole lot faster.

There are so many variables to consider and many have been listed. I have
had many tripods of the years and many are collecting dust. Just picked up a
carbon fiber model, light weight but not cheap.

It is a tough decision on what to get, and the best way is to look at
various models and pick the one that will do the job for you and you can
afford .

www.alldigital.fotopic.net

It's really hard to get an "all use" tripod without paying a small
fortune for carbon.  Stability generally = weight at all but the
highest cost factors.  I use a relatively heavy and clunky Manfrotto
when the gear is going in the car and a lightweight and very compact
Cullman Magic II for traveling by bicycle and long trips.  The Magic II
just won't work all the time but it's better than having nothing.  All
the lighter tripods can be made more stable by adding weight to the
center.  I made up a 12"x12" nylon drawstring bag that I fill with
rocks when traveling light then hang it from the center.  The Magic II
center post is removable and can do double duty as a monopod of sorts
as well.  The real money should go into the head for your main tripod.
Get something smooth and easy whatever style you decide on.  I like a
ball head that gets replaced by a real simple rotating disc for
panoramas.

My rule of thumb is that the lighter the camera the heavier and sturdier the
tripod needs to be, as the weight of the camera will help stabilize the
whole thing.  You can have a very rigid and lightweight carbon fiber tripod,
but something has to keep it on the earth.  A decent wind and you'll be
running after a very lightweight tripod.  Also, I like a head that lets you
easily flip the camera for vertical shots.

Fancy heads are only if you need them, and look for a tripod that has a
comfortable grip at a nice working distance from the head.  Futhermore, look
at the feet.  The combo rubber feet with recessed spikes are nice (no extra
parts).  The spikes work well on really thick carpet, too.

I really appreciate all the responses.

Not to beat a dead horse about which tripod(s) are best for which
situation -- a couple of points that haven't been mentioned yet:

   1)    I strongly recommend you get a tripod head with a quick release
base -- put one plate on your camera based and one on the 70-200 2.8L foot.
Believe me, will save a lot of your time and the plates are relatively
inexpensive.

   2)    Very important -- get a tripod that when legs are fully extended
and with the head, quick release plate, and camera) that puts the viewfinder
on your 20D comfortably at your eye level.  You can always squat if you put
the tripod lower but not if it's above eye level and I suspect the vast
majority of your shots will be with the tripod legs extended and at a
comfortable eye level.

   3)    Get the remote release for the 20D -- there's so much emphasis put
on the innate stability of the tripod -- which is important -- but many of
the vibrations/camera shake that occur is because of the photographer
hurriedly depressing/mashing the shutter release which induces movement
through no fault of the tripod.

Mike