I see various soldering stations marketed as being designed for lead-free work. What's different about them? I like the Weller WESD51, but I see no mention of it being suitable for lead-free soldering.

There are several differences between lead-free systems and regular soldering irons. First, lead-free solder tends to operate at a slightly higher temperature, as high as 230 C, as compared to 180 C for leaded solder. A tip temperature of at least 350 C is needed. For most (well made) soldering stations, that's not a problem. However, the stability of the temperature control becomes a more important issue since heat damage is more likely. Generally, "lead-free" soldering irons have better temperature stability.

The second issue is the tip coating. Lead-free solders tend to be much more chemically aggressive, and can even eat through the walls of hot solder pots that were designed for tin-lead solder-- so it's a good idea to think twice before just replacing leaded solder with lead free. The tip life is also shortened-- significantly-- just by operating at a higher temperature, and lead-free tips are (or should be) designed to last longer under these circumstances.

Finally, the lead-free soldering stations are manufactured without lead in them. That's obvious, perhaps. However, if you use an older soldering station (which has had lead on the tip) to solder an otherwise lead-free system, you are introducing lead into it. Better to be careful, eh?