1) I was recently looking at compact tractors. To meet the standards, new tractors are fitted with Diesel Particulate Filters. Even if they don't flat-out fail (which happens too often, and costs thousands of dollars to fix), these filters require "regeneration", which is code for "a light comes on on the dashboard, your tractor stops moving, and runs wide-open throttle for 20 minutes while you stare in bewilderment". Google Kubota B3350 Regeneration if you want lots of fun horror stories.
2) Diesel commercial trucks. My uncle drives a large cube van over the road. When his truck goes into regen mode, it doesn't require wide-open throttle like a compact tractor, but it does lose power and tops out at 40-50MPH. If you are on a freeway where the flow of traffice is at 80MPH, this is dangerous and a major problem.
3) Diesel pickup trucks: Many trucks, in an effort to avoid additional hardware above and beyond the DPF, have taken to injecting fuel during the exhaust stroke. The idea is to have the unburned fuel get pumped through the engine and on to the DPF, where it burns and increases the temperature to the point that the carbon burns out of the DPF. Problem is, when you inject fuel into an engine during the exhaust stroke, it dilutes the oil in the cylinder, reduces lubrication, and causes premature wear/engine failure.
In the two cases I am familiar with, diesel motorhomes and diesel compact tractors, the value of used vehicles has increased quite a bit in response to the fact that all of the new ones suck. My Dad owns a gravel pit, and he (along with everyone else in the industry) buys and re-builds existing vehicles from the frame-up to avoid having to purchase new products, which are unreliable, sometimes dangerous, and uneconomical to maintain. Google "glider truck kits" -- it's basically the idea of purchasing all the parts of a new vehicle (except the powertrain), and then manually taking the powertrain from a worn-out truck, re-building it, and installing it into the glider kit. This is often done at an expense that is similar to that of purchasing an entirely new vehicle.
Many equally horrible solutions to this problem are employed. Some designs incorporate a fuel burner directly into the exhaust design. Others require a separate tank of catalytic fluid that must be filled regularly. One thing that all of the designs have in common is poor performance and piss-poor fuel economy. Many vehicles from multiple manufacturers have had trouble with catching on fire due to emissions control systems.
If the EPA wants to make tough rules that will force manufacturers to produce products with the best available technology and encourage development of new technologies, that's one thing. When the EPA makes rules that result in dangerous and/or non-functional product designs, I think it's time to back off just a bit until the technology has come closer to being able to reasonably achieve the regulated cleanliness levels. It's completely insane to make a truck that will force you to slow to 40mph while you're driving it, or to make a tractor that will force a farmer to take an unplanned break in the middle of the day (remember -- farmers work all day long, and daylight is a limited, precious commodity to them).
EDIT: To be completely clear, if you are doing work with the tractor that can be done at full RPM, you don't necessarily need to park it. However, if you're doing something that requires lower RPM (using a PTO attachment that needs low RPM like a post-hole digger, or a spreader, etc ... or doing low-speed work on a tractor with a geared transmission), then it's time to go get some coffee. Personally, the two major things I want to do with a tractor are run a large snowblower and maintain our horse pasture. For the horse pasture it's not a big deal, but if I need to clear the driveway before leaving for work in the morning, an unplanned 20 minute wait is completely unacceptable.
