Differences between all the type of servers?
Retail-Like(low/mid)
Customized(low/mid)
Retail-like(high)
Old-Style PvP/Instant
Faction
Highly customized(low/mid)
Highly customized(high)

retail-like low/mid/high is LINEAGE, rest are sh1ts and I wont comment..

Basically low-rate retail means it's the way it's meant to be played. High-rate retail usually resumes to how pvp is meant to be played.

Customized servers can be all right, there are alot of servers with minor modifications such as rebirth system or tattoo system. But for highly modified servers, the owner really needs to know what he's doing in order to keep balance. L2gold was probably the server with the highest degree of customizations. Apella armors were custom, there were l2gold weapons and buffer classes were boosted, so that they could get self-buffs that are 1 lvl higher than standard buffs (for example prophet could get might lvl 4, blade dancers all dances lvl 2, warlords had thrill fight boosted, and things like that.
This was also the only BALANCED custom server that I've known. It's been reborn as far as I remember, but not too many players on it anymore. C4 really is kind of an old chronicle.

1) is obvious
2)npc buffer,gmshop,class changer for all 3 classes,free subs and etc
3)smth like raidfightpvp/lym/inferno - sticks to retail gameplay but could have customizations which do not dramatically affect retail gameplay(its hr afterall)
4)old style is old style!mppots,everything in the gmshop,as many buffs as u want and endless skill spamming (like c4 era!)
5)custom items,farming grounds etc
6)same-difference in rates