In short my thoughts:
I didn't think the movie was very consistent, some scenes were great, and others were a huge drag. It seemed like they were trying to repeat Skyfall with all the different locations, but unlike Skyfall not all of the locations were very interesting; even though Rogue Nation filmed in two of the same countries and seemed much much more interesting while doing it.
* Overall, the story seemed half-baked. Or maybe they threw most of the right ingredients in, but rushed putting it together, accidentally adding salt when the recipe shouldn't have had it.
* The main villain reminded me of khan, a shoehorned re-invented villain.
* The main henchman, played by Batista was fun.
* Mendes did a great job directing certain scenes, but some locations and scenes were boring. It was a mix of good and bad, slow and rushed.
* The storyline was poorly organized, and had too many coincidences.
* Monologues slowed down the movie. It wasn't paced very well, and was very flat and lacked an interesting climax and resolution.
* Scenes just didn't seem right, everything was either rushed, or too drawn out.
* The plot got stupid.
Spoiler version below:
I thought M's storyline was interesting, and I wish it were the complete focus for the first part of the movie.
A terrorist attack makes the UK consider using C's anti-terrorist company in order to protect the country. M and C clash, and there is suspicion cast on C - perhaps because of the convenience of terrorist attacks and C's company moving in.
James Bond should have been investigating C, uncovering his more sinister plot. Blofeld should have been shrouded the entire movie. Bond would slowly uncover that Spectre exists through C, and not through some other guy... Randomly. I didn't like how M and Bond's stories were separate and just coincidentally linked up.
The next part of the movie would have Bond traveling to Austria where he would directly confront Spectre member Mr. White - who he should know was linked to this organization. Extend his stay in the Alps a little longer, everything important from Morocco to here, and extend the train scene; make him meet and interact with Lea Seydoux more here; take inspiration from the film North by North West and From Russia with Love. This is where Blofeld should have been based, he should have been an Austrian Noble, and his organization should have old roots.
The final portion would have been Italy, when all the villain people met up. C would be the main villain here, with his criminal intentions now fully in the open. He should have come off far more dangerous, and used some of his drones in attempts to assassinate Bond. Maybe working with Spectre to cause a terrorist event in Rome for the purpose of getting them to run to him for anti-terrorist help; or they could have just relocated to Vienna like Rogue nation did. Blofeld should have remained shrouded, and perhaps had C executed himself; he didn't need to take a fall in this movie - assuming they are bringing him back later.
Making things personal between Blofeld and Bond was stupid. Blofeld being Austin Powers' brother... errrr, Bond's brother, was even stupider. And having him arrested at the end was just weird.. like Khan in Star Trek.