I watched through all of Spartacus last year and loved it. The recast because of Andy's death was jarring but I got used to it (Naevia's recast less so). I started off hating the actor who played Gannicus but I quickly grew to like the character, especially after he comes back later on. He was just really believable as "the best gladiator out there."
I started watching The Arrow a little over a week ago and am up to mid-S3 right now, when Captain Boomerang appears. I was reading this thread earlier, saw a pic of Ashur, and then captain Boomerang appeared a few moments ago. I watched episode way back when S1 of Flash was airing, since it was a cross-over, but didn't recognize the actor again when I started watching Spartacus. He's definitely memorable enough now that I'll recognize him in anything else. Waller looked familiar and I couldn't place where I had seen her until this thread when I saw a pic of New-Naevia. Manu of course I recognized right away. He was awesome as Crixus and is great as Slade Wilson.
All of these shared characters led me to look up a link between the shows and it seems like the Arrowverse shares some writers with Spartacus. Liam was Weather Wizard in Flash and I didn't recognize that either, but I see it now.