So since the Dressrosa arc is basically over I'll give my thoughts here:
- Great villain. One of the best I can think of. Crocodile is still my favourite, but Doflamingo is on a close 2nd together with Akainu.
- Gear 4 revealed. Much better than I could have imagined.
- Enjoyed the introduction of all the characters a lot
- Green bit part was great
- Colloseum part was great
- Fujitora filled a new role we haven't seen an admiral have before
- Sabo was introduced (succesfully). Didn't interfere too much in the story, yet still had an important role
- Interesting and unique subordinates of Doflamingo.
- I liked how Dressrosa was explored. From "this is a beautiful but mysterious island" to "this is a dark and terrible place"
- Bringing Law along gave defeating Doflamingo more importance and new cool team fighting (yet Luffy still went 1v1)
- I enjoyed the part with the gladiators vs the henchmen of Doflamingo as well as the invasion of the 'mountain levels'
- Robin finally got some impact in the story!
- Sorry but the whole thing with Rebecca was miserable to read through. Most of her story was just waste of space
- The royal family were in my opinion too passive during the end of no return.
- Cipher Pol was a disappointment
- A bit too many flashbacks. I can't remember them all, but something like Kyros/Rebecca, Law, Doflamingo (all three major) and Chinjao, Sabo (two minor). Too many!
- Poor top dogs of the family. Pica, Diamante and Trebol were too weak if you ask me.
- I would have liked to see Robin, Zoro and Franky be tested a bit more (as in they have to reach their limit)
- Kyros was a disappointment. I expected differently from when we first heard of him in the colloseum.
- I would have liked to see Sanji have more impact.
- I didn't like the invasion of the palace in the beginning. Wasn't any exciting as the experienced reader already knew that no big fights would happen yet.
- A bit too many factions involved at the same time (without any clear objective)
- I absolutely despised the copy ability of Doflamingo
- I would have liked to see Viola's ability to be used a bit more than just for reporting information to King Riku.
- Gear 4 made the final battle against Doflamingo a bit too swingy
How Bellamy's purpose in the story will benefit the overall rating I have yet to decide
So the final rating will be 6/10. Better than Thriller Bark and Fishman Island (if we only include the main arcs), but not much better than that.
Actually, my thoughts aren't far from yours, but I'd rate it higher. Maybe a 7'5/10, but I need a final re-read. I guess the positive elements outweight the negative for me. Like, I liked all the flashbacks, I never cared that much about Doffy's clones and I know Cipher Pol will have their spotlight later on, so I wouldn't list those as negative elements.
I am though annoyed by the way Trebol went down, how Viola's ability was underused or the Manly Man Man thingie. I'd also add the way Oda lenghtened the final fight from 786 to 789, both chapters included, or the one too many chapters of gladiators shouting "it's gonna be me!", "no, me!", "no way, me!" while beating poor defenseless random runts before the officers appeared; these are the only occasions where I thought things could've (and probably should've) been shortened a bit. Or how difficult it seems for Oda to fully focus on a fight. I honestly think chapters like 785 are ultra awkward because of that: Doffy sporadically (and a bit anticlimactically) awakens his DF, and when it seems round 2 begins. . .we cut to a long King Riku speech, only to cut back to Luffy Leo Bazookaing Doffy against the side of a mountain. Things like this kinda hurt the overall "feel" of the manga. You get into fight mode, then sudden cut. You get into arousing speech mode, then sudden cut. For the future, if I could talk to Oda about just one thing, that's the one concern I'd tell him about: try to focus a bit more.
Other than that. . .yeah, insanely ambitious arc. Re-read awaits when it's all over, and this time it's gonna be definitive.