You can tell that James Cameron was trained as a physicist. His movies have an attention to scientific and technical detail that is sadly lacking from most Hollywood movies, movies like Terminator 3: Rise of the Machines. They are replaced by more car chases, explosions and pretty girls. Some of the big concepts in Cameron's movies may be flawed (time travel paradoxes, the water technology of the aliens in The Abyss), but it's the attention to realistic detail that mean you don't end up saying "yeah, right!" throughout the film Sadly, the is not true of T3 (guns and buckles are attracted to magnets too!).
That is not to say that T3 is a bad movie. I was quite tense throughout, even if it is predictable at times. On its own, it is an above average action flick, but it does not, and should not inhabit the same universe as the previous movies.
What impressed me so much about Terminator 2 (I saw it 3 times on the big screen) was not just the amazing special effects, but also the feeling that the characters' actions mattered. They weren't just standing by, they were out there trying to save humanity's future. Fate is what we make it, was the message, and it is a good message. Sadly, it's a message rejected in the sequel.
Also missing is the wonderful humanisation of Arnie's robot self that was a major theme in T2. Now he just a two dimensional character spewing out one-liners. His female opponent lacks Robert Patrick's natural menace, but does a fair job. Unfortunately, the same could not be said for Marco Beltrami's entirely unmemorable score.
T2's finale was emotional, triumphant and final. T3 is a movie that should never have been made.