Personally, I found the story to be by far the best thing about ME3. Well, up until the last 10 minutes, but that's something that you'll have to decide for yourself.
Combat - is slightly more refined than ME2. The cover system works a bit better. Melee is more satisfying, and guns now have attachments, or perks to improve their performance.
The voicecast, well, is the voicecast, and again, they're excellent.
The disappointments for me were the lack of hub worlds, the simplication of side quests into 'go fetch' missions, and the massive amount of auto-dialogue.
It's a strange one is ME3 - it has some of my favourite bits from the series, and some of the worst.