With a whole new race coming, and one new unit for each old race, there's a lot of things to be looking forward to.
Personally I think the IG Kasrkin squads and the SM Chaplain are going to be the coolest new units. In every RTS squad I've ever played, I'm always a sucker for special forces or elite units, so I'm really looking forward to sneaking a crack team of Guard around the map and such. I'm guessing they're going to be the Marine killers of the IG infantry. Secondly, Chaplains have the potential to be awesome. In the fluff and TT they're fierce CQB warriors, always belting out the fury of the Emperor vocally and physically. I think in the middle of a battle hearing the Catechism of Hate would add just that much depth to the game. I hope they really focus a lot on the Chaplain being able to have special abilities, like the Librarians WoTE and such. Also, they just look downright badass.