Well, first off, each version does provide improvements. However, it doesn't occur to
all asset types... Not all built-in content may take advantage of the new features either.
Some examples might help here. Note, these are examples, not complete lists.
TC3 introduced new steam physics and sounds (neither are easily back dated to earlier versions), as well as new functions for steam locomotives (smoke, bogie settings, etc). Also added a setting for the intensity of the 'high beam' (default) headlight on locos.
TS2009 was the 'big' improvement in a sense. It introduced new splines, a new system of LOD for scenery, 5m grid for layouts, new materials for objects. Again, each of these means backdating is not possible...
TS2010 had some 'big' change for content as well. Again, maps had changes, namely the 'layers' system. There is also the seasonal assets functionality, and of course speedtree assets. Note, TS2009 SP4 allows TS2010 routes to work in it, except where content is missing. Note, does not have speedtree functionality.
TS12 had slightly less in the way of content changes, but introduced multiplayer instead of 'content' functions. However, it did still include some improvements. The biggest (outside of the multiplayer) is the new sound engine which supports doppler. This requires that diesel loco sounds (as well as other sounds that are setup for diesel locos) be upgraded, and hence won't directly work in earlier versions (that said, it is only the addition of a few tags). TS12 (and the TS2010 MP beta) introduced 'self shadowing' system (yes, objects with shadows can cast onto themselves), albeit in TS12 with a triangle/index/vertex limit (currently effectively 1333 triangles).
It should, however, be remembered that you really aren't going to see much in the way of improvements to older content simply by adding new functionality. Only content built to take advantage of the new functionality will see these improvements. A good example is the 'tbumpenv' material (TS2009 SP1+ only), which adds a 'sky' reflection onto the model (controlled per pixel across the texture), as well as a normals maps, and a specular map (normals being supported since TRS2004). The addition of these, and some light weathering (personal opinion on the light weathering

) can really bring a model out, although having the model stand out isn't always a good thing, and can mean making more models to go with it so that it blends in...
I would be interested in how you were able to transfer TS12 content to TS2010, since you don't appear to have TS12 registered...
As to content, not all of this was 'freeware', an example being some of the content from JointedRail in TS12. Much of it is also unreleased content, in particular the routes. TS2010 did also include some new routes, although there were also 'upgraded' versions of previously released routes included as well.
It should be noted that we are currently looking for content creators who are interested in having their content included in the next version of Trainz (please see the appropriate announcement).
As to what else was done between TS2010 and TS12? Well, we did make improvements to the performance in Trainz (many of the comments we see are that it either performs the same, or better).
It should be noted that, if the computer is below the minimum requirement, then it is unlikely to run well/correctly. The minimum requirement is the specification that Trainz is expected to run on with all settings on minimum.
As to how well, this can depend entirely on the software side, if the computer meets the system requirements. For example, a very 'bloated' computer (full of temp files, unnecessary processes running, hard drive not defragged regularly, etc) can give very poor performance, whilst a very 'clean' or 'lean' computer (regular cleaning of temp files, very few processes running, regularly defragged hard drives, etc) can give better performance. Dust inside the case (causing overheating) can play a part as well, as can the ambient temperature in the room (a simple pedestal fan to circulate air can make quite a difference - known from experience there

).
Most computers sit somewhere in the middle of the above though, and hence could be said to get 'average' performance for their system specs.
As to giving your old Trainz versions to a friend, we do outline in the End User License Agreement (and on the serial number registration page) that Trainz licenses/serial numbers are non transferable. As such, we don't support transferring serial numbers between accounts. Please remember that you agreed to the EULA when you installed Trainz...
Please excuse the long reply, I wanted to cover much of this to explain how things are going.