As I'm not interested in shedding grams (a diet would be more helpful for me), I'm leaning towards a steel frame. But steel frames are often cheaply manufactured, and the price is slowly starting to reflect trendiness than quality.
How do I recognise a high quality new or used steel frame? Are there simple things to look for when weeding out the poorly made or overpriced frames?
First, when you say you are looking for a steel frame, what you probably mean is a ChroMoly Steel alloy frame. This would be a frame made from tubes where chromium and molybdenum were added as the steel was refined. Frequently you will see a sticker on the tube with a number like 531, 631, 725 or 853 which is used to describe the particular blend.
Second, spotting a high or low quality frame has become more difficult over the years as machining and welding techniques have filtered down. With older frames you definitely want to look at the quality of the welds and brazing, as well as the paint. Welds may be smooth, or have a regular pattern. Watch out for welds that look rough, 'globby' or irregular. It used to be that the use of lugs was a hallmark of lower quality frames, but today you see them more often on high end boutique or custom touring frames.
You may also want to feel the bottom sides of the tubes for a "seam". On older low quality frames you frequently find rolled tubes, where the steel was a flat sheet that was then rolled and welded length-wise.
Finally, the durability of a steel frame is largely a function of the paint quality and maintenance. Water and rust are the primary enemies of a steel frame. If buying used you will want to closely inspect the area around the bottom bracket, dropouts, and the whole underside of the bike for chips, cracks and visible rust. For long life, as well as the low environmental cost, steel v. aluminum is probably a wash. In the US and most the developed countries both aluminum and steel recycling is around 50% and the production costs and impacts are pretty similar. And while corrosion may be an issue, an aluminum frame won't rust.
Tubing, as other have mentioned, higher quality frames are made of higher quality tubing. High quality tubing will often be a tube set from a "name brand" and have the appropriate stickers (e.g. Reynolds 531). High quality tubing is typically double, or even triple butted - i.e. the tubing has a larger internal diameter, and hence thinner walls in the middle of the tube. Low quality tubing may have a weld running along its length if it is made from rolled stock.
Lugs, the presence of lugs as opposed to welds is not an indicator as to whether the frame is high or low quality. Lugs have been used in high end steel frames for a long time, but other methods have seen surges in popularity (e.g. welding). What you want to look at is the quality and refinement of the lugs - do they look like something that has been meticulously machined and crafted to fit the tubes just right, or do they look like something that a plumber would use to join to pipes together. High quality frames often have special patterns added to the detail of the lugs for aesthetic reasons.
Rear dropouts, a technique that is often used on really cheap steel frames is to have the rear dropout pressed from sheet steel and the ends of the seat and chain stays just "squished" around the ends of it. A quality frame will have a well crafted junction between the stays and the dropout. Like lugs, builders often put some artistic flourishes around the dropouts.
Welding/brazing, this should be even and consistent. There should be no small gaps that could trap moisture and cause rust. You shouldn't see small bits of excess metal on the frame around the weld or braze. The surface of welds should either be a regular pattern of beading, with each bead around the same size, or filed smooth. There should be no pitting of the surrounding tube or lug from the welding or brazing.
Componentry, if you are looking through second-hand bikes to find your quality frame, it is also worth looking at the associated componentry on the frame. Usually high quality frames have been built up into bikes with high quality components and vice versa.
Finally, consider the fact that there are many low quality frames out there that do the job just fine as a bike. High quality is not a necessity for a frame to build up to a perfectly usable and long lasting bike.
If you want the best for you in a steel frame, look into local framebuilders. There are newer tubesets by Columbus and True Temper that individual builders can use to make awesome frames. Furthermore, they are competitive with aluminum frames (yes they weigh more, but the difference is far less than how overweight 'I' am...ymmv). Here in the Seattle area we have Davidson and Rodriguez bikes built locally. Your area probably has someone who builds high-quality frames as well.
There are treatments for the inside of steel frames to prevent corrosion, and any reputable steel frame builder will have treated their frames.
On a bit of a tangent, but have you considered a cardboard bicycle?
This would only meet one of your requirements - a low environmental cost. It certainly wouldn't last 100 years, but might have less environmental impact over 100 years even if replaced several times as it's certainly easy to recycle.
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