The two games are vastly different. The most alarming similarity I think would be the similarity with the five playable races. In FFXI they're Elvaan, Hume, Mithra, Tarutaru, and Galka. In FFXIV their respective counterparts are Elezen, Hyur, Miqo'te, Lalafell, and Roegadyn. There are some variations - for instance Galka have tails and Roegadyn do not. But the FF11 versions generally seemed to serve as the inspiration behind a more refined corresponding race in FF14.
Other similarities include the avatars/primals, magic spells, jobs (all seven of which are in FF14 are also in FF11 along with thirteen others - and two more coming to FF11 next year apparently). Like the races, these jobs and their "artifact gear" look different between the 11 and 14 versions as well.
Others will draw comparisons between places like Windurst (one of the cities in FF11) and Gridania (the forested city-state in FF14) but a lot of that is left to interpretation.
As with most things there are similarities and there are differences. The developers have had roughly a decade to pump FF11 full of lore and numerous expansions have given the game a lot of different layers and made the world a much larger place, where FF14 is still relatively new and sometimes feels a little bit more clunky and restrictive by comparison, though I believe this is what the ARR will be addressing.
I'm sure there is a lot more that can be added to this. My own opinion is that the gameplay overall is much smoother and less complex, but I realize that may also be because I spent much longer getting used to it. FF14 sometimes feels awkward to me. The concept of "classes" in FF14 kind of graduating into the jobs we've all grown to know and love in many FF games is one I still find a tough pill to swallow at times. And the lack of variety in that regard is often frustrating. That is to say when there are only 7 jobs, and so many having leveled all of them, it's difficult to feel unique. In FF11 there's much more variety.
Hope this helps answer your question. Without knowing more specifics, I'm left to make kind of generalizations and express some of my own personal opinions. I like both games on equal levels, but I seldom play FF11 any longer.
EDIT: Also the worlds of Vana'diel and Eorzea are quite different as well. Vana'diel feels more medieval overall but has smatterings of references of things like ancient technology and such. Eorzea at times almost feels a bit more steampunk and technology is a bit more widely distributed.
Other similarities include the avatars/primals, magic spells, jobs (all seven of which are in FF14 are also in FF11 along with thirteen others - and two more coming to FF11 next year apparently). Like the races, these jobs and their "artifact gear" look different between the 11 and 14 versions as well.
Others will draw comparisons between places like Windurst (one of the cities in FF11) and Gridania (the forested city-state in FF14) but a lot of that is left to interpretation.
As with most things there are similarities and there are differences. The developers have had roughly a decade to pump FF11 full of lore and numerous expansions have given the game a lot of different layers and made the world a much larger place, where FF14 is still relatively new and sometimes feels a little bit more clunky and restrictive by comparison, though I believe this is what the ARR will be addressing.
I'm sure there is a lot more that can be added to this. My own opinion is that the gameplay overall is much smoother and less complex, but I realize that may also be because I spent much longer getting used to it. FF14 sometimes feels awkward to me. The concept of "classes" in FF14 kind of graduating into the jobs we've all grown to know and love in many FF games is one I still find a tough pill to swallow at times. And the lack of variety in that regard is often frustrating. That is to say when there are only 7 jobs, and so many having leveled all of them, it's difficult to feel unique. In FF11 there's much more variety.
Hope this helps answer your question. Without knowing more specifics, I'm left to make kind of generalizations and express some of my own personal opinions. I like both games on equal levels, but I seldom play FF11 any longer.
EDIT: Also the worlds of Vana'diel and Eorzea are quite different as well. Vana'diel feels more medieval overall but has smatterings of references of things like ancient technology and such. Eorzea at times almost feels a bit more steampunk and technology is a bit more widely distributed.
"One of the deep secrets of life is that all that is really worth doing is what we do for others." Â ~ Lewis Carol
Eva's Journals  |  Eva's Wiki Page (coming soon)  |  RP Handbook
Eva's Journals  |  Eva's Wiki Page (coming soon)  |  RP Handbook