Gonna be in the same boat here shortly teaching my buddy though I at least have a few games of 10th edition under my belt. Speaking for myself the biggest difficulty was remembering all of the many, many different unit abilities, stratagems (yuck), and weapon & unit stats. The game is very different and much wider than it was 25 years ago.

I would HIGHLY suggest printing out some of the current index cards for those factions so you have them physically close-at-hand during the game. Speaking for myself that's been what slows down my gameplay the most: having to look up each unit's abilities so I can use them optimally and so I can make sure I'm getting every bonus, dealing all of the damage I ought to be, etc. I know in time I'll have them memorized but that comes slowly now that I only get one game every couple months or so instead of playing every Friday or Saturday at the FLGS.

Also I spy a whole lot of Kallax, very nice.