Forum rage comes from the fact that a lot of people on here have only played FFXI and FFXIV as their only MMOs and either want FFXIV 2.0 to be FFXI HD or stay how 1.0 is and are afraid of changes or modernization of FFXIV. There is some people who hate the archtypical WoW type MMO who want FFXIV to have nothing in common with that game.
Here the most common things people like to rage about:
1. Jumping - A lot of ragers hate jumping well because it wasn't in FFXI and WoW has it. They think their game is so realistic that people shouldn't be jumping, ragers like to use their own buzz words like 'bunny hopping'
2. Grass and other background graphics - This one I don't understand guess people love nit picking over the smallest things.
3. Backpedaling and Strafing - Standard MMO movement, people seem to rage on this because they once again don't like the animation or play with the controller and others don't like the idea they could be at a disadvantage compared to KB+Mouse users.
4. Regional Servers - A lot of rage came out on this topic because a lot of FFXI players liked the world server set up, now that servers will have a lable on them people some people fear they won't get to play with all their friends from the world. While others are like the idea due to the fact they will have less lag and play with people who speak their language.
5. Faster Battle Pace - A lot of rage on this is because once again FFXI players were use to hitting 1 WS when their TP hit 100 and prefered the slower pace of combat that was in FFXI and FFXIV. Ragers on this topic like to throw out words like they are making the game 'a spam fest'. Most modern MMOs are much faster paced then FFXI and FFXIV.
6. Removal of character animation. A lot of fluff animation was removed, I.E. starting and stoping animation in favor of a more cleaned up and smoother animation. There is a lot of rage about this because its a change and some people liked the all the character animation that was in 1.0
7. Questing as a form of exping - So much rage when SE released questing videos. Some FFXI purists don't like quest based leveling because they grinded lots of jobs in exp parties and did the same thing in FFXIV and think everyone should do it, reasoning ranging from it helps people learn their job to they make friends on the server will doing exp groups. Solo based quest leveling has become a staple of MMOs once again this was set by WoW and a lot of former FFXI players can't stand this. As it stands the general MMO playing population much perfers quest based leveling over grinding.
8. Quest tracker and helper - So much rage came out again this time due to the fact SE showed they added a quest tracker. Ragers kept saying things like 'holding peoples hands', 'babymode' 'making the game to easy'. When in reality quest tracking is a useability feature to make quest based exping go faster and is in every modern MMO. Usually quest tracking can be turned off with a toggle in the menu UI however that isn't good enough for most ragers because if you give the player an easier option they will take it. FFXI gave very little directions in their quest once 1 person figured it out it was usually posted on FFXIclopedia where the majority of the playerbase looked at to figure out how to do their quests.
9. Cross Server Content Finder (dungeon/raid finder) - A lot of rage came out because of this. Ragers argued that it would once again make the game too easy "they lose their special snowflake status if casual gamers could get into raids easy and get gear that was once accessible to the hardcore", they think lose a sense of server community, they don't want to play with strangers from other servers. This feature was invented by Blizzard in WoW and since then copied by a lot of MMOs thus once again a lot of hate since it was originally a WoW feature.
10. Gay Marriage - Weird that this comes up in a video game.
I am sure there are many more raged filled topics but these are the ones I see that stand out.