not bad but could always use some treatments.... 
Depends:: and Depends. If you're talking 'bare room' without anything in it, you can make it go either way. Lots of windows? Flat ceiling? Willing to install / tune room treatments?
http://www.marktaw.com/recording/Acoustics/RoomModeStandingWaveCalcu.html
above would be good place to start. You'll see that any room dimension of 1.5x another will produce same frequency results. this is standing waves which result in loud/boomy or 'suckout' or lack of a specific frequency....some people will complain of 'lack of bass' in this case.
At over 5000ft3, it is a fairly large space. If you like it LOUD, you'll also need more juice or more sensitive speakers. Don't rely on a single sub, you'll need at least a PAIR, located asymmetrically.

Dynamite room,can do alot, I can't help,but you will do it with some R&D on the computer,its all here.....good luck,Bob 
A great room for some big dipoles...!
Dave 
Dave, are you saying that good bass will be hard to achieve in this room?
Thanks,
Adam 
This room is an excellent size. See my system I am about the same size. You need treatments though but nothing really needed for near reflections of midrange/treble but you will need mostly stuff to tame the bass and mid bass  think large broadband bass traps and anything else you can get in there to absorb (plush furniture, bookshelves...etc. etc.). The modest sized space will mean that bass modes are very audible as everything will be well separated in time  unfortunately all the space will make remaining modal issues more noticeable as you have overall less masking. 
Adam, The 20x30 is close to the Cardas ratio that really works. 1.6 is a turn key winner every time. Pay attention to how normal conversation sounds; is it comfortable to have an extended conversation? These dims should respond to proper setup without having to resort to the ugly stuff. In a perfect world, your speakers are a third of the way in and your chair is at two thirds. 
Adam
No, bass will be very good after a few room treatments. I'd start with the corners, and rear wall.
I was just stating that large, deep, wide rooms...are a dipole speaker owners dream. They offer everything that, that breed of speaker needs to really excel.
Dave 
Does the Cardas website discuss the origin of 'there' ratio? Hint: the Cardas logo...a nautilus shell, is the clue. 
I considerate it excellent. Provides great space to work with. 
For anyone with a fascination for numbers and curiosity about what George Cardas and others are promoting.
Read "The Golden Ratio, The story of PHI, the world's most astonishing number" by Mario Livio, Broadway Books, C 2002 (available in paperback).
This book covers the golden ratio from when it was defined by Euclid, through 2000 years of historical understanding. This ratio appears widely in nature and and has been reported used in architecture, painting, analyzing the stock market, and now the design of speaker cables and optimal listening rooms. Many examples and pictures of PHI in action.
1.61803... is a remarkable and mysterious number. 
My math is weak.Is 1.61803 half of Pi?Your name Z also is mysterious.A coincidence?Destiny?Stay tuned Egyption lovers.....metaphysically speaking of course...Happy Halloween,Bob 
If a room sucks you'll never get it right. Even with lots of room acoustic devices (bass traps, diffusors). If the room is quite good (proper acoustic characteristics), then you can optimize it further. Caveat: room dimensions don't guarantee that you'll have good acoustics, even if you follow the golden ratio (whatsoever) rule.
Chris 
With all due respects to Euclid et.al. the Golden Section goes back....Way Back. Many Egyptian monuments, including the near 5000yr old Great Pyramid use this ratio as a design element.
There IS a mathmatical relationship between phi and pi. Phi is the only number that squared = itself +1. (2.618) other relationships exist. 
It goes back father than that. Nature has been using the "Golden Ratio" to design things pleasing to the eye, and to function their best for aeons. ie: As mentioned, the Nautilus shell, what we consider the most beautiful faces, the human hand's bone structure, your ear's cochlea, the DNA helix, flowers, etc.(http://cuip.uchicago.edu/~dlnarain/golden/activity7.htm) 
Magfan, writers/historians studying the Pyramids disagree on whether PHI was used in the design. According to Livio, it is possible either PHI or PI was used and more likely it was PI (see his book for the explanation). Just because there is a relationship between Pyramid elements that approximates PHI doesn't prove it was used. There is lots of controversy surrounding PHI.
A regular pentagon is made up of 5 isosceles triangles. The ratio of the longer side to the base is PHI. It is an irrational number, that is, it is a number that cannot be expressed as a ratio of two whole numbers. This really bothered early geometrical researchers.
PHI can also be constructed by taking the limit of the ratio of adjacent numbers in the series formed by summing integers:
1+1=2 1+2=3 2+3=5 3+5=8 5+8=13 8+13=21 etc.
Series is 1, 2, 3, 5, 8, 13, 21 etc.
2/1=2 3/2=1.5 5/3=1.666667 8/5=1.6 13/8=1.625 21/13=1.6153846... etc.
A logarithmic spiral has the property of not changing its shape as it grows larger. The radius keeps increasing as the spiral grows just at the right rate and this is governed by PHI. This spiral is the logo for Cardas. Does PHI work in the design of cables and rooms? George clearly believes it does. 
Just a slight correction........ The RATIO is 1:1.618. It is NOT the NUMBER 1.618. The RATIO is often called the Golden Mean and it is found both Geometrically (The diagonal of 1/2 a square added to the square), and mathematically as Zargon has demonstrated. This mathematical progression is called the Fibonacci Series after the Latin mathematician who discovered it. It works by adding the 2 preceeding numbers to obtain the next in the series and if you move far enough into the series (55/89) you get 1:1.618. 
Zargon, I wish I was near my reference materials rite now! I think it was Strabo or Herodotus who mentioned that the ratio of side to area of base was 1/2......I simply can't remember. Bottom line? Phi was used in calculation:
Also, Pi=6/5 phi2.
Yes, there is Substantial disagreement. However, and this is WAY beyond a HiFi posting, the numbers/ratios of such monumental constructions simply don't lie. Way TOO much to go into here. PM me for more information, some of which is nonstandard. I have met several professional Egyptologists and they typically, though very smart, would have trouble balancing there own checkbook. 
Halcro....we're both right. While phi is derived from the fibonacci series, and the further you go, the better the resolution of the number, so it IS a 'ratio' It is also an irrational number.....like pi, which some simplify to 22/7 for ease of calculation. While I know the simple way to derive phi, I know of no such way to derive pi. Is there one?
That such a ratio can have so many uses and applications is a miracle of nature and the universe. Look up, 'The Monk Drunk on Wine' a book from the middle ages. 
Adampeter, sorry as we seemed to have hijacked your thread. It started with a comment that your room dimensions seemed similar to the Golden Mean. I don't believe they are, however, if so, that would have meant that there was good reason to believe room modes would be fairly evenly distributed.
I think the size of your room is a positive for creating a large sound stage and image. You will be able to keep the speakers well away from reflecting boundaries and will have the space to move them to maximize bass performance. Every room has its challenges, but large rooms have more flexible solutions.
Do not worry about the concrete floor. Many great listening rooms have them, and you are avoiding the possibility of floor resonance and footfall issues on a TT. A thick carpet is just fine and will minimize high frequency reflections and help damp reflections.
I happen to believe there are many ways to design a good listening room and using the Golden Mean is one of the ways to approach good bass performance. I have done exactly that currently in my own room and it works extremely well. I don't know the limitations of your space (fireplaces, doors, windows etc.) but if you have the option to shorten the room with a new partition, here is one to consider.
Starting with 30 x 20 x 8.5
Go to 30 x 18.5 x 8.5 and fill the 1.5 ft space behind the rear wall with damping material. Put speakers about 3.5 ft in front of the short wall about 8 ft apart and 5 ft off the side walls. This is very close to what I had in a previous room which worked well.
If you are interested in this approach, it would be helpful if you described your room conditions in more detail.

Magfan, You are correct, it was Herodotus who is credited to have said "the pyramid's base is square, each side is eight plethra long and its height the same". Translations of this statement have been reinterpreted by many, and somewhat twisted to say "the square of the pyramid's height is equal to the area of its triangular lateral side." If so, one could argue that the pyramid was designed such that the ratio of the height of its triangular face to half the side of the base is equal to the golden ratio. Not only is this a stretch, it fails because the actual pyramid measurements do not come close to those implied by the statement. 