I think one way to make CAPTCHA be more acceptable is to make it funny, but for some reason most CAPTCHA do not do this. Would this be seen as unprofessional or could this be a good way to make a necessary annoyance more engaging and amusing?
I think Captchas should be simple. The user has no interest to decode cryptic pictures. But I don't know how secure are some simpler captchas...
Examples for simpler Captchas: http://serie3.info/s3capcha/demonstration.php
The CAPTCHA used on the UX.SE(and most likely other SE sites) is brilliant. It is light hearted and gives a chuckle instead of annoying you. This might not appease everyone, but it sure attempts to make light of an annoying situation. Also while being funny, it gives context to why you are being asked to enter the CAPTCHA text, to prove "I'm a Human Being".
It also goes on to explain on the right side:
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