I'm personally a fan of super-heavies, but in my experience the super-heavy vehicles are very fragile compared to gargantuan creatures.
EDIT: I'm actually quite a fan of 7th edition over all, the problems with it seem far lesser than the ones that older gamers have told me of from 4th edition and earlier (I started in 5th). The only real problems I have are related to assault rules and some of the psychic powers.
Assault rules wise I'd like to see the role of assault grenades inverted, rather than allowing units to charge through cover with no initiative penalty I'd have them knock the defending unit to initiative 1 as they're recovering from the grenade blast when they get charged, while charging through cover would have no effect on the assaulters other than reducing their charge range. I'd probably also have assault grenades prevent overwatch.
Psychic power wise I'd like to see a more consistent level of power between them. As is telepathy, malefic and divination are all incredibly potent while telekinesis and pyromancy are basically useless. I'd prefer the weaker ones to be lifted up to the same level as divination, though malefic and telepathy (invisibility specifically) could do with some nerfing.
Obviously some codices need updated and given formation/special detachments and so on as well to match stronger 7th codices, but that's not a problem with 7th as much as it is with codex writing.