When the OpenGL spec is updated, they only ever add features. So in theory, the latest and greatest hardware with support for the Core and Compatibility profiles should run super old OpenGL1.1 code just fine. This has turned out to be true. I've spent 12 months learning OpenGL1.1 and have a fair grasp on it.
I had a read of a few chapters of one of those fancy new OpenGL4.2 books. Almost all of the features that I rely on have been deprecated (like Display Lists), which lets me assume that there are better ways of doing all these things.
Lets consider that 1.1 is likely to be supported, in full, by ALL modern hardware. 1.1 was released in 1992. I'm not coding the hard way just to support 20 year old PCs. :-p I think its reasonable to assume most gamers are running hardware that bottoms out at about 5-year old mid range.
I think the newer methods are designed to universally be either one of two things: better performing, or easier to code. I read somewhere that its never both though! XD
What version of OpenGL is most widely supported by ~5ish year old hardware? What version makes most sense to use, given these considerations?