Other than difficulty, how different is EO1 to 4? Specifically the class/skill system?
A lot of small tweaks here-and-there for general usability and flow of gameplay (e.g., Battle Speed, side-stepping, interface in general). A lot of mechanics differences, with notable things like Retirement functioning differently (Stat bonus based off the class retired), Resting costing way more (10 Levels), and the Skill system being ... awkward (the Anti Elements get worse with more than 5 points*; skills were fairly inconsistent in their growths**).
There's a few other things like bland encounters (I honestly can't tell you what most the bosses did beyond "They hit you.") and the least forgiving dungeon. Many of the classes aren't that great (sorry Ronin, Hexer) and status effects/binds in general are pretty hard to use (sorry DH).
*Technically, they still improve. It goes Damage Reduction (1-4) - Immunity (5) - Absorb (6-10). The problem being that the Dragon breaths did Damage AND something else. 5 would stop everything they did while every other rank would not. Not even the Absorb ones.
**For instance, here's a few random skills:
Code:
0x87: Fire _______________ Lv5 _______________ L10 --- _______________ L15
TPCost (01): 4 4 5 5 6 6 6 6 6 6 --- 0 0 0 0 0
BaseAt (15): 10 10 10 10 10 10 10 10 10 10 --- 10 10 10 10 10
Power (02): 80 85 90 100 105 110 115 120 125 130 --- 135 140 145 150 155
Acc (04): 200 200 200 200 200 200 200 200 200 200 --- 200 200 200 200 200
Speed (03): 60 60 60 60 60 60 60 60 60 60 --- 80 80 80 80 80
Code:
0x25: Apollon _______________ Lv5 _______________ L10 --- _______________ L15
TPCost (01): 8 9 10 11 12 12 12 12 12 12 --- 0 0 0 0 0
Power (02): 300 310 320 330 390 400 410 420 430 500 --- 510 520 530 540 620
Speed (03): 20 20 20 20 20 20 20 20 20 20 --- 40 40 40 40 40
AilChc (12): 0 0 0 0 10 15 20 25 30 40 --- 45 50 55 60 75
Acc (04): 200 200 200 200 200 200 200 200 200 200 --- 200 200 200 200 200
Code:
0x16: HP Up _______________ Lv5 _______________ L10 --- _______________ L15
HP Up (32): 110 112 114 116 128 130 132 134 136 150 --- 160 170 180 190 200
On the bright side, we have this information so you can make these better choices so there's that ... I guess. But, for instance, Fire/Ice/Volt and many damage skills have that pretty basic +X% per point that never changes. Then you have many skills like Apollon which have small increments per point and then suddenly gets a massive spike at Level 5/10/15. And then all the Passives are pretty much the same thing with the 1/5/10 spikes.