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To answer the original question, Weber introduced a new 18.5" WSM in November 2008 for the 2009 model year. It added a lid thermometer, larger water pan, more sturdy legs with heat shield, redesigned access door, and a tweak to the charcoal grate that closed two gaps on either side that allowed charcoal to fall through.
The water pan increased from 1 gal to 2.5 gal because many people using water or sand as a heat sink were using larger Brinkmann charcoal pans as a replacement water pan in the WSM, preferring its 2 gal capacity. Problem is that in typical fashion, Weber repurposed an existing grill part as the new WSM water pan. The depth of the new pan tends to interfere with piling up charcoal and smoke wood in the charcoal chamber. BBQ champion Harry Soo has covered the heat shield with foil and used it as a replacement empty pan instead of the stock pan when cooking without water. So the edge goes to pre-2009 WSMs when it comes to the water pan design, in my opinion.
Lid thermometer is controversial, not in the sense of whether it's needed, but as to the quality and accuracy. I prefer cooking with a lid therm vs. remote probes, but if you like remote probes then the lid therm matters not to you. I would test the lid therm and if not fairly accurate replace it with an aftermarket model or ignore it.
Legs were made wider so that they would be sturdier. Good improvement. Also retooled to hold the heat shield. Not sure why that was added other than to satisfy someone in the legal department. Yes, the bottom of the bowl does get hot and the heat shield helps protect surfaces below...but so does common sense.
The door was redesigned so that it hangs open from the middle section when opened. The old door would simply fall off the cooker when opened. The old round knob handle had no indicator of when the door was properly latched, so doors thought to be latched might fall off mid-cook, runaway temperature ensues, meat is ruined. New handle visually indicates the closed position. However, it's got a more complicated mechanism that can be problematic. Overall I think the new door and handle are a plus.
In Fall 2013 for the 2014 model year, Weber made two improvements to the cooker. One was the silicone grommet allowing pass-through of probe thermometers into the cooker. The other was adding handles to the bottom cooking grate.
I don't think there are any fit or finish or material issues that pertain to year of manufacture. Unfortunately there are some parts shipped that are out of round, primarily middle sections and charcoal bowls, but that has always happened over the years.
P.S. I documented a lot of this in this article:
Weber Bullet History
