I've recently been working on the eco-renovation of two buildings in a conservation area, one of them listed. This has been a real challenge - historical buildings like these two are designed to allow the free flow of air (and with it heat energy) through the building fabric. Alter this and at best you will get condensation problems and at worst the building fabric will rot. The only way to do it effectively is external insulation and vapour protection, but this will change the appearance of the building and you can't do that in a conservation area.
This gets even worse with the listed building. All windows have to be preserved where possible, and if replaced, then replaced like for like. You can now get double glazing with the same bead size as old single glazed windows, but this is still verboten due to the different depth of each unit which you will notice if you look carefully. Secondary glazing can be put in, but you can't draught proof the outer pane or you will probably trap moisture between the two with fatal results.
Having struggled with these constraints for the last couple of months, I am now of the opinion we really need to think again. There must be a better balance between preserving our heritage and making buildings suitable for the 21st century.
If you think this is a minor issue, then just think - 5% of existing buildings are listed or have some form of conservation protection. As these are the least likely to be demolished due to that protection, they're also the most likely to survive into the next century.