Nah. The difference is one of (learning) rigor. Intellectual and scientific. You learn a lot in the process of getting a Ph.D. about how to think, how to analyze, and how to research.
That sounds awfully generic. It doesn't sound that different from a Master's (heck, maybe even an undergrad) program that requires a thesis. Yes, there's a difference in the sense of supposedly doing novel research, but I'm not sure the degree to which you're theoretically moving the state of the art forward justifies your time investment in many cases.
It's a matter of degree. A _typical_ undergrad thesis is more spoon-fed from the perspective of "here's the idea, go do it". A typical masters project is still more advisor-generated than student-generated. A typical Ph.D. project is more student-generated than advisor-generated in many cases. Please add copious "not all"s to this paragraph as the variance is extremely high and everyone's case is unique. And the rigor expected increases substantially. Again, generally.
The major thing of a Ph.D. isn't really how much it moves the state of the art forward, it's how much it moves you forward. You will almost certainly approach research and your area fundamentally differently after.
That's true for me as well as a professor with respect to research: I'm pretty convinced that my largest contribution is the Ph.D. students I've mentored more than the chunks of research I've done. I'm proud of the research. But I'm more proud of the students. They've gone on to do wonderful things.
The thing it hangs on is "justifies" - financially, a Ph.D. in CS is hard to "justify." Intellectually, it's easy. Depends a lot on your individual utility function! I could have dropped out of my Ph.D. program to go be a pre-IPO employee at Google or Akamai and, ah, let's say that my financial picture would have a few extra zeros on it compared to where I am now. :-) But I'm really glad I didn't. It's been a very fun journey and I've never felt like I'm just doing the routine. That's worth a lot for me.
I'm assuming you don't have a Ph.D. if it sounds generic to you. I think it's really impossible to explain to somebody who hasn't gone through it how much a Ph.D. changes the way you think.
Whether it's worth the time investment is another matter, however, which I'll leave alone.
You don't really learn how to learn in a masters or undergraduate. You learn how to consume information, but not how to research new things, validate those and defend it to others in a rigorous way.
Whether it's worth the investment depends on ones goals in life.
If undergrad doesn't teach you how to learn at at least some level, that probably makes me question some of the value in getting an undergrad degree at all as that's one of the arguments for going through undergrad. I won't argue that a PhD isn't something of a next level but obviously there's a big cost involved, especially if PhD-type research isn't something you'll be doing.
