Starting Northrend at 68 is completely fine if you know what you're doing. Yes you're going to fly through Outland with barely an upgrade or two but the gear hike from Outland to Northrend isn't as bad as Old World to Outland for your other alts.
All trees are viable for leveling but if you find yourself wasting runes on deathstrikes, pick up blood talents for a little added self healing.
Rune tap, vendetta, and butchery are simply the best soloing talents for DK's. You should rarely need rune tap and never need deathstrike.
http://www.wowhead.com/?talent=hZZxcezAxoxkrIRt
Oh and stay in blood presence.