When I started in this business, at the very beginning of the replacement window's invention, things were a lot different. Replacement windows started out at 2 5/8" thickness, now they are pretty much universally 3 1/4" thick. When we started there was a filler strip to fill the gap found in most installations to the parting stop. Replacement windows were all aluminum, and there weren't many choices. You could get mill finish single glass double hung aluminum, or you could get mill finish single glass double hung mill finish aluminum. Straight aluminum windows were a problem due to condensation and energy loss but still an improvement over worn out wood windows, thermal breaks were developed in the mid 1980's to separate the inside of the window from the outside to improve thermal properties of aluminum windows. It certainly was an improvement, but still left a lot to be desired as far as thermal peromance.
At the beginning there were no horizontal sliders, no casements or any other variations like garden or bay or bow windows. Then in the early 1970's painting of aluminum extrusions was developed and color was added. At one point my previous employer had 13 different colors available, from white, which has always remained about 80% of sales, to pink to green. As the industry developed, slowly, options became important. Color mainly, but the windows evolved in quality and features, storm windows were mounted to the exterior of the aluminum prime replacement window for better thermal qualities and progress continued.
Insulating glass was developed in the late 1960's or early 1970's. Then triple glass was developed in the mid 1970's. Low Emissivity or Low E glass was developed and became available in the mid 1980's and new and better coatings on the glass surface were developed allowing better and better U value performance. The Low E glass available 20 years ago had far less energy efficiency than what is available today. Glass coatings are probably the biggest improvement in the thermal properties of windows.
Vinyl windows were developed in the mid 1970's. They were pretty much universally not very good products then but they offered far superior thermal properties than aluminum so the development and learning continued. One of the early products was basically and aluminum prime window completely encased in vinyl to improve its thermal properties, still not great, but much better than straight aluminum. High quality and high energy performance vinyl windows started to evolve in the 1990's with the glass improvements continuing till today. The technology has continued to evolve in both vinyl extrusions and glass coatings making todays products far superior to what was available even ten years ago.
There are other materials that companies use to frame windows with; wood, wood composites, clad wood, and fiberglass, are the most prominent, but to be frank, there is no better framing material for windows than vinyl. It's gone from nonexistent to clearly the most popular material in a couple of decades because it is the best material. It is thermally efficient, strong, resilient, impervious to rot and mold, and no other material can beat it.
So if your windows are more than ten years old. And your home is drafty or chilly in the winter and uncomfortable with "hot spots" in the summer. You should investigate replacing your windows with new highly energy efficient, high quality vinyl replacement windows. The saying goes "your paying for new windows whether you actually get them or not in higher energy bills", and that's not to mention the comfort, security, curb appeal and peace of mind new windows will give you. Time is of the essence, investigate now.