For reference, there are four generations of the Fun series (excluding Fun1, Fun1.5, and SuperFun).
2004-2006 - 1st generation
2007-2009 - 2nd generation
2010-2014 - 3rd generation
2015-present - 4th generation
The pictures below are of the 2nd (yellow), 3rd (blue/green), and 4th (green) gens 2Fun.
For additional reference, pull up pictures online of the Fun1 / Fun1.5 - those are based on the 1st generation design.
Here is consolidated feedback based on my experience and that of my kids:
2nd gen was a huge improvement over the 1st gen. Better balanced stern, better river runner, much improved outfitting, still has plenty of play.
3rd generation was a nice improvement over 2nd gen. Knees were brought in, the design was more playful overall, outfitting was further improved in 2012.
4th generation - biggest design shift in the series - better river runner and faster than previous 3 gens, my boys think it's more forgiving and also less playful than previous gens.