The Count wrote;

GIMMICK AGE - (1990 - 1999) - a total waste of paper, absolutely nothing good came out of the 90's. Majorly freaking stupid ret-cons were tried, characters were forever damaged with idiotic changes, and gimmicks like company-wide super-crossovers, foil covers, reboots, #0 issues, #1/2 issues and #1,000,000 idiocy polluted the stands. No wonder the industry fell. It should never have tried to get up.
I would agree with what the Count said with just a handful of exceptions. I thought JIm Lee's X-Men during this period was very good. A couple of the Image titles were pretty good as well. But my question is, the Count points out that during this period that some characters were forever damaged with idiotic changes. I think a couple of these that stand out in my mind are Barry Allen's Flash running himself into a cinder and Aquaman losing his left hand. Those two changes were like saying that The Flash and Aquaman were disposable characters. Another biggie that comes to mind is when Batman's back was broken. Then along came Kingdom Come and we saw that the back injury had lifelong effects on Batman. Nonsense.

My question is, what change do you think has been the most damaging to a character? Feel free to be brutal in your assessment.