It's hard to narrow things down so much, but if I had to pick three things...
To love:
1) BLU! One of the most versatile jobs you'll ever find anywhere, given a proper gear setup and such. BLU was my second job to level, right along with DRG back during the "lolDRG" days. Never got a party but once in a blue moon, but I never faltered, and reached 75! Since then, it's gotten better, and I've been working hard to be one of those BLUs that can accomplish almost anything. Draylo's solo of both ACP and MKD storyline bosses still inspires me to work harder. D:
2) In-game versatility - between the number of jobs we can play as well as the unique abilities and spells each come with, I'm rarely at a loss for interest in a job. BRD got a bit annoying as I fell into casting Double March/Haste/Cure IV and nothing else, but for the most part all of my jobs I've leveled - 12 currently - have all been very engaging in their own ways.
3) Storylines! I fell in love with CoP and WoTG (the latter moreso!) and ToAU was amazing as well. Abyssea felt lackluster in its storyline, unfortunately, but for the most part, I love the adventure that accompanies the face-smashing that we do on a daily basis. :3
To Hate:
1) Spikes. I hate you spike spells, you are the bane of my fast-attack jobs like NIN and THF. Shock spikes go off and bam, I'm taking 3-6 hits of spikes to the face, stunned, and as soon as I'm unstunned I'm already attacking again. Memories of Turul raping me with 300/hit shock spikes still haunts me to this day.
Blaze spikes get some leeway, they only hurt.
2) Head-in-the-sand players. By this I mean those who assume that they know everything, and that when you offer advice they tell you off or say "my way works better still" despite you proving to them, before their eyes, that it's not. I'm not saying you have to change your whole style of playing, merely offering a more effective alternative, geez. x_x
3) Full Aurore/Teal players. Full Perle at least is a reasonable start, especially with the haste bonus, but full aurore has better options that you can get pretty easily. Same with full teal; most of the pieces are situational. And if you're wielding a level 20 weapon at the same time... I'm going to glare at you until your brains melt. D:<
...I've run into a number of these, if you can't tell.