How is it that Skynet is actively targeting Connor in Salvation, fully aware of both his future centrality to events and his childhood escape from their robot assassins? The established timeline does not support this. Armed with this knowledge, why would Skynet even commence with their eventual plans to attack the Connors in a different time, knowing as they must that such plans will fail? Another question: Why am I putting more thought into this than the screenwriters did?
To add on to that point -- since the machines shouldn't even know that Connor is important, it is even more outrageous that they know that Reese is important. If Skynet knows Reese, Skynet knows everything, and the entire franchise is stupid and pointless. To accept Terminator Salvation on any level is to create a black hole of stupidity that disintegrates and swallows up the franchise in toto, and that simply cannot be tolerated.
Just as the T-800 in T2 tells John, "I know now why you cry," I can say that I know now why Steve continually denies the existence of T3. Personally I thought T3 had enough good moments to outweigh the bad -- sure, there's some timeline discrepancies with John's age and some dopey failed jokes, and the general feeling of a franchise being stretched too thin... but there is also the pleasure of revisiting something familiar, a few memorable action scenes, an interesting arc for John, an effective, if depressing, ending, and some successful comic moments as well. Overall, there is enough to recommend it as a watchable, if inferior and not-wholly-canonical entry in the series.
Terminator Salvation, however, should not be allowed in the door at all. There's just nothing that good in there; the action is flashy but utterly unmemorable and uninvolving, Sam Worthington and Anton Yelchin do their best to connect, but at no point does the story reward them or us or give anyone a reason to invest in the characters, and the ending is just plain retarded, as was the equally retarded ending it replaced. Certainly nothing here good enough to justify overlooking the staggering crimes of logic and story.