Is there any consolidated guide to what changes were introduced to various models year-by-year? In software products, there are usually release notes that tell you what changed from the prior version. I'd like to find something like that for Harleys.

Trying to figure our what changed year-to-year, and what models are affected is pretty frustrating. For example, one review mentioned that the ABS in the 2014 softail was considerably improved from 2013 -- or was it the suspension.

When evaluating bikes on the used market, I am finding it very difficult to compare bikes of the same models, a year or two apart.

 

You are asking what is known as an open question - in other words there are endless answers, which it isn't practical to try to write down. You don't mention which models you are asking about, but if say you are looking at buying either 2013 or 2014 Softail models I would recommend you ask in the Softail section, where you will find more experts to help you.

 

I'm not sure what is wrong with an open ended question in a discussion forum.
When asked, basically the same question, in another forum, the most useful answer I received was: check the online reviews of the motorcycles you are considering, for the particular years you are looking at. Here is an example, this review of a 2014 Heritage, it tells me exactly what changed from 2013: http://www.totalmotorcycle.com/motor...ailClassic.htm
And here is the 2015: http://www.totalmotorcycle.com/motor...ailClassic.htm

And though there doesn't seem to be any single "consolidated guide," this advice gets me 90% of what I am looking for.

BTW.. the "review" I was referring to in the initial post was one of those youtube, video reviews, the written "magazine" reviews are much better for what I was looking for.