Kale can still be used in moderation - just so long as its not a staple, it can still be part of a varied diet to help get a better range of nutrients

I think many people have heard about its binding properties and freaked out, which has given it this image of being dangerous to beardie health.
Any lettuces are generally more water than nutrients and not of much good in the diet - although again they are good supplements to a diet.
I think its
www.beautifuldragons.com that has a "good/bad" list of veggies. Staples should be things like collard/spring greens, mustard or turnip greens (if you can find the things for sale! I never have!), carrot tops...though of course as many different things as can be mixed up (bar the lighter coloured lettuces like iceburg) the better the diet will be. I tend to use the collards in every meal, varying radicchio, escarole, frisee, chard, spinach, kale, squash, carrot, rocket, watercress, beans and various fruits from meal to meal. Dandelions are meant to be awesome, as has already been mentioned, but I have yet to use those...I may have to go weeding in the warmer weather!