The Delta recruits get nice bonuses, but from my vision, it's mainly a means for newer players to get more on par with current players and to help out along the grind. However, there are bonuses you get from leveling up the Delta recruit that are account wide, most popular being traits.
I deleted my old Klingon that collected an obscene amount of "dust", and created another one for a Delta recruit. Personally, I'm hoping on it when bored and taking my time with him through the stories and leveling. It's a joy for me, because when I leveled up a Klingon before it was during the dark days when you basically had to grind out the same missions over and over for XP just to level. So being able to play that side and experience a more personal story with it has been nice.
Other then the Delta recruit, there are just a few highlights of Season 10. We got 2 new story missions for the Iconian story arc, 2 new space queues, 1 new ground queue, another Reputation to grind out levels for new traits and gear (well worth it, being on par with Omega's Rep level of value IMHO), and the new sector revamp (which is BEAUTIFUL).
Not too much to get lost in, but enough to add some more fresh air to the lungs of STO. I'm sure G, Davis, Hau, Ryo or someone could point out the over-looked revamps to PvP if there are any to weapons and such, but it's something I've not paid attention to so I cannot provide an answer to that depth if it interest you.