To answer the OP:
Yes, you will be behind the curve for not playing FFXI, be prepared to learn quite a bit in the beginning it will be confusing for everyone that hasnt played it, but FFXI experience definitely does help.
If you just get in the mindset that the UI was intended to be used with a controller and is keyboard centric, not mouse, that will help get you used to the interface, and in the long run I find this to be one of the only games where keyboard alone > keyboard + mouse.
Understand the difference between guildleve "quests" and plot "missions". There was a distinction made between quests and missions in ffxi, missions were critical to advancing plot and in some times character advancement, quests gave small rewards, were occasionally repeatable and gave rep, missions were major and quests were commonplace. Missions were very difficult to know where to pick up if you weren't looking at some knowledge base online, they start by talking to one of the MANY NPCs under the proper conditions.
Classes/Jobs/Levels: In ffxi you had job levels, your character could switch jobs (class) at their house and it was a lot like switching characters in other games, you still had your rep and missions you completed and stuff (some physical attributes) but the overwhelming majority of your attributes were totally switched when switching jobs. ffxiV kept the concept of having different jobs on the same character at different levels (switch on the fly by changing your equipped weapon) but added in a physical level to give you some common attributes that you dont lose when switching jobs (a level 75 white mage wont be switching to a level 1 monk and be basically at ground zero in this game like they could in ffxi)
Combat is similar, but faster in ffxiv, it still seems to be mostly one at a time, and a monster that is an even match to you (at your same level) is really an even match... no grinding out 3 or 4 guys at once your same level or higher, think one at a time, lower than you and watch yourself while you do it (mobs in ffxi and ffxiv are "meaner" for their level than most mmos)
Crafting: You will be looking for recipes online, in ffxi this was the case, its also the case in ffxiv... crafting is very similar in concept, crystals or shards are used in conjunction with a bunch of materials, you select all the mats click the craft button and in ffxi it attempted to make whatever that combo yields, in ffxiv it does the same only its not automatic it goes into a minigame. You can find ppls suggestions/strategies on the minigame yourself, but the concept is that you mix materials are not told what it will create until you finish, and recipes are not collected for you, you WILL be looking online for recipes if you craft.
Theres a lot more, but yeah playing ffxi is a definite help, its not even close to exactly the same, but imo its bananas to think that they're as different as say ffxiv and wow... things (a lot of em) will carry over despite what you're being led to believe... I hope this helps...