Sony V Canon/Nikon lenses

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Its more a question of why are Canon and Nikon the more sought after brands.

Canon and Nikon are the manufacturers of choice for professionals. As you can tell from these forums, the occasional fan boy crops up and tries to convince people only those two brands (better: the brand of his choice) is the one to go. To most, the availability of a $7k 200-400 mm zoom is not crucial since we won't buy this lens anyway.

Sony seems to be a new player in the market after the acquisition of Minolta and appears to have desirable features such as zeiss lenses and image stabilization, however, what I was more interested in was insight on value for money, durability, reliability, lens availability and overall quality (pros and cons on the brands). Things that I would generally look into when buying say a car, a tool, a surfboard...etc.

Sony has bought Minolta, a company that has decades of experience in building cameras and lenses. Sony manufactures sensors for Sony and it is not very surprising that many sensors Sony uses are related to the ones Nikon uses. That should give you some indication on Sony's expertise in that area.

Are you meaning to say that a Sony camera is just as good if not better than its Canon and Nikon counterparts?

Yes, Sony bodies are comparable to their Canon/Nikon counterparts.

As a rule of thumb, newer cameras have an edge in image quality. But honestly, if you're working at normal ISO values (ISO 100~640), they are imperceptible/non-existent (ISO 100-200) to practically irrelevant (higher). As I wrote earlier, your Sony's sensor is more advanced than what your friend has in his 30D. That hasn't stopped it from producing images that have impressed you.

Unless you have very special needs (e. g. if you shoot pro sports or need a tilt-shift lens), Sony offers you a full lens line-up, just like Canon, Nikon and Olympus, for instance. Sony does not try to compete with Canon and Nikon head-to-head (which I think is a smart move). They have recently introduced the A55, for example, a camera with a very advanced AF system, a fixed mirror and an electronic viewfinder. Hence, the camera is able to do 10 frames per second, a feat normally only accomplished by Canon's and Nikon's top-of-the-line pro sports bodies (the 1D and D3s).

If I were to stay with Sony, can you recommend what I might look for in a used minolta lens of good quality say in the 200-400 focal range?

Well, what's your budget?
I just checked my local dslr forum, there is a Tokina 80-400 mm on sale for €250. If you want something rather long that is literally built like a tank with good image quality, that might be an option. You can get Sigma's excellent 100-300 mm f/4 for ~€550 (this lens is a gem in Sigma's line-up).