I think the Vietnam War would have ended pretty much the same way, only sooner. Democrats would not have been as free to abdicate responsibility for the mess they created in Vietnam as easily under a Democratic president as they did under a Republican president.
We would not have had the bold foreign policy moves that Nixon made. A Democrat would never had been able to open relations with China the way Nixon did, so that would have been a detriment.
Other than that, I'm not sure there would have been a huge difference. Humphrey would have continued to pursue liberal policies that had failed, but Nixon didn't do much to turn these back anyway.
It's very hard to say.
You forgot one big thing; no Watergate. Surely that would have been a major plus?
The long term effects that has had on the national psyche continue to be very important to this day (distrust of organizations and institutions, political and non-political alike).