Spore appealed to the artist and world-builder in me. Its replay value comes in me being able to go back again and again and make lots of critters and the entire world they live in. I had no problem with the changes in gameplay, because I found it neat that a game integrated so many genres into itself.
That said, I think it was just too easy. But it wasn't meant to appeal to the experienced gamer in the first place -- and I think that's why people here don't like it. If you play RTS, the tribal stage is ridiculously baby-level compared to, say, Starcraft (to pick an RTS just about everyone has played). If you played Civ and really got into it, the Civilization stage just isn't going to do it for you, because it's simple. Spore appeals to Sims players who honestly just want to create critters and make them do stuff. This is fine. It just shouldn't be expected to fulfull the role of ultra-strategic RTS plus revolutionary civilization game plus Eve-Online space world, etc. I didn't have those kinds of assumptions, so I'm happy with what I got.
I do agree that herbivore is a path to total destruction and really binds your hands. Then again, if you didn't want to kill other creatures, well, you got what you wanted. Everything knows you're an herbivore and tries to kill you, much like in real life. I haven't played a game where diplomacy is a better/more fun way to do things than killing everyone, except maybe Alpha Centauri to a minor extent.
Then again, I just like killing things in games. ^_^ My allies betrayed me too much in Centauri anyway.