Standard milspec forged recievers (with the exception of those made by Mega) are all pretty much the same. Differences include fit and finish, and what shade the anodizing is done. Examples of poor quality lowers include poorly broached magpwells (too tight or loose, leading to feeding problems) out of spec trigger pin holes. Honestly for the most part, its all about the looks.
Check this out.
If you want a little some more, check our forged Mega (Rainier Arms, Riflegear etc) lowers. They usually have bigger flared magwells, tensioner screws to remove upper/lower play, and some come with a cool retaining screw to hold in the takedown pin spring.
These are little things, nothing major.
Now billet that's another story. There are some pretty major differences there. Structurally, billets are inherently weaker than forged, but they can be made to compensate and even be stronger (tho heavier) than a standard forged. Besides being able to really change the way the reciever looks, they can do things like, intergrate the trigger guard, add amdextrious mag and bolt release controls, reshape the magwell, strengther the reciever exention area. and I'm sure a bit more. Check out AXTS, Mega, Seekins Precision, etc etc.