I think the main flaw is that the two stories (Marcus Wright/John Connor) are not woven together well. At some stages the movie would flick back to what was going on with John Connor and I would be surprised that he was still in the movie!
I read some reviews which rightly point out how surprising it is (given the legacy of the previous movies) that it lacks strong female characters.
All in all a bit of a damp squib.
The current IMDB ratings for the 4 movies are:
Terminator 1: 8.1
Terminator 2: 8.6
Terminator 3: 6.4
Terminator 4: 6.7
That's about right.
And I see that Arnie is working on a fifth movie! No mention of James Cameron unfortunately.