A few thoughts after playing this weekend:
I still am not 100% sure I am going to play this game on release, but just in case I:
1) Leveled Thaum and Arcanist to 20 for quicker BLM access, Arcanist feels more like an affliction lock than a summoner to be honest, the pet does everything for you and you just throw up DoT spells
2) This sort of relates to Yoga's comment about pulling aggro, in order for an arcanist to maintain highest dps in an instance you need to keep your Dot up on every mob, but it seems tanks don't have the capabilities to hold aggro if you do this. Maybe they just don't have their abilities yet, but it is what it is. By that same token I'd rather pull aggro with my DoTs so the healer can heal, I noticed a lot of tanks can't even get aggro off the healer.
3) I picked up mining/goldsmithing, I assume all gathering professions are as boring as mining. You just run around for countless hours right clicking or am I doing it wrong? None-theless it is pretty horrible, I think I liked fishing in ffxi better than this gathering system getting 125 xp per hit needing over 10k exp around level 10. It is surprisingly faster to level than I thought, but still very grindy. I find it surprising to be this grindy when the leveling system has no grind at all.
4) The storyline/class quests are still incredibly annoying, they requiring you to run back and forth across zones (yes you can teleport, but seeing how I don't know how much GIL I will need down the line I try to avoid using a 200gil teleport every time I need to travel. I like the story, but the running around is a bit excessive for level 15/20, I'd imagine at higher levels this will only get worse.
5) Instances are pretty good, I noticed they offer excellent exp if you do them when you are equal to the mob level. When I was 17 in Copper Bell i was getting 500-700 exp per mob and chaining, with a good party I can see this being better than FATEs for geared alts.
I tried to touch on as much as possible, it is a very good and polished game, there are obviously a lot of question marks about how everything will be late game, but only time will tell. I have to decide now if I want to continue my adventures.