GTA is an OK game design but i don't know why people wet their pants over how breakthru it is - it's no more open-ended or free ranging or innovative than 6 Dozen OTHER games i've owned over the last 2 decades.
If you didn't get it from playing the game itself, I'm not sure how I can explain otherwise to you.
But I'll just say that I bought GTA3 on a whim, when it first came out, and I was amazed by it. No hype involved. It was just extremely entertaining. There's a lot of features that I could point out, but apparently you think that Ultima Underworld has already pulled them off (I'm not going to even begin to try and explain why that's horribly, horribly wrong).
GTA's basic concepts aren't all unique, but they are extremely well executed. A lot of effort has been put into the details, and it shows. It's only been getting better since GTA3. They've got a great formula going for them.
My guess would be that maybe you're a late-comer to the GTA series, and that you've been overhyped to death by your friends and the media. Who knows.
That's all I got to say 'bout that.