CD1,
I own two rifles that I think would match your requirement; reliable, accurate, affordable to shoot, light, high capacity, and semi auto. One of them is better for a strictly home defense situation, and the other has different benefits.
The first is a SAR3. This is an AK74, but in a .223 caliber. Many people on different forums will give different comments, but the newer ones have apparently solved the problems that the originals had. This rifle meets all of the requirements listed by you, unless you require true MOA accuracy (which I don't think is required for true home defense). It is not a rifle that you will admire for its workmanship (with it's stamped receiver), but it works very well.
The second is a AR15. Mine is a 24" Bushmaster with the ribbed barrel, so it is not as handy as the SAR3. However, it is easily capable of sub MOA accuracey. Many options are also available should you need/want them.
Both of these use the readily available, cheap .223 ammunition, and have high capacity magazines available (not always cheap, though). This means that you can practise with these a lot.
As for comments, unless you live in the country and have plenty of room around you, I am not sure in what situation you would need these. As you used "TSHTF", if everything went REALLY bad, these would both be excellent rifles to have, as the .223 is the military standard, and almost everyone seems to have ammunition for them.
However, an older military rifle, a true "battle rifle" might be an even better choice if things were REALLY, REALLY bad. The .308 M14 clone would be a better all around rifle, IMHO, or even an M1.
The M1 uses 30/06, probably the cartridge that has taken more game in this country over the last 100 years than all others combined. You can get one for $400 or $500 if you don't want a collectors quality gun. The M1A will cost more, but has the benefit of using the round that is probably the favorite sniper long distance round in this country.
Sorry about rambling on, but you did ask for opinions. I own all the above (except for the M1A), so I guess I REALLY have things covered!
Casey
[This message has been edited by Casey (edited October 20, 2000).]