I have levelled up a few classes now and so far mage is my favourite, 175hours in game at the moment.
Levelling is OK from 1-9 then it felt like a bit of a struggle. In groups your great but alone its a nightmare, I was in full heavy, have around 1400 HP and mobs like the highwayman raiders (lvl18ish) can hit me for 700dmg on their specials and 200 normal attacks, these are same level mobs. Unsure what its like in light armour, but I am sure its not great.
Mage needs another form of CC, doesn't need to work in PVP necessarily, that can be used when mobs resist frozen.
On the above note, the chilled/frozen resists feel terrible, alone its a death sentence in a group, specially with other mages it feels terrible as none of your talents work reliably, you cant set up shatter procs and cant refresh combust. There needs to be a debuff you give mobs that acts in place of chilled/frozen to proc these abilities that doesn't CC mobs. I can't stress enough how awful it feels playing in a group just to see resist, resist, resist, I may as well just spam abilities off cooldown.
The whole thing with mages is AOE, combust needs a passive that spreads its damage in AOE.
The theme seems to be frost-lightning-fire so shattering shards makes more sense to proc off burning.
There needs to be a meteor type ability, which could tie into the above CC, If used on a Volatile target stuns/trips. AOE not a huge cast time.
Last but not least, I think rewarding you for cycling frost-lightning-fire would be nice with a passive that increases damage or casting speed by 2% per rotation up to 10% would feel pretty good too.
All the above is my thoughts on PVE mage and how it feels to me. Abilities obviously will get tweaked and changed throughout the way but these above make it feel well better, the main one being the resists which ruin the feel in group play.