I changed my pads yesterday, but I didnt bleed the lines, I just pushed the piston back with a C-clamp, and I forgot to take the reservoir cap off . My brakes still feel the same (my old pads werent that bad, the rear pads were making noise). Its just that the brakes feel a little spongy, a mechanic told me that my rotors are pretty bad, he pointed out "little cracks" too, all I could see was the lip... he said that the rotors were probably the problem. Is this common on a 29k mile Z? Previous owner said he never changed the pads....

Did I do any damage by not opening the reservoir cap?
Should I bleed the lines and get new rotors?

 

I doubt you damaged anything by not removing the cap. If you had damaged something there probably would have been a noticable change in the feel of the brakes or brake fluid leaking from somewhere. I also doubt that you would need new rotors after only 29k miles. I have over 60k and my rotors are fine. You shouldn't need to bleed the brakes if you didn't open the system and let air into the lines, but it could help put your mind at ease. You said that the brakes feel the same as before, this leads me to believe that nothing is wrong with your car. I don't know your mechanic or how trustworthy he may be, but he could just be trying to sell you some work.