So I did explain this in the guide, but maybe not well enough. Lets hop on over to the wiki and pull out some numbers:
http://wiki.project1999.org/Ambassador_DVinn L20 / 600 hp
http://wiki.project1999.org/A_ghoul_executioner L35/1575 hp
http://wiki.project1999.org/A_ghoul_sage L37/1740 hp
http://wiki.project1999.org/Kobold_priest L40/2000 hp
http://wiki.project1999.org/The_ghoul_lord L47/6500 hp
http://wiki.project1999.org/Froglok_ostiary L50/8750 hp
http://wiki.project1999.org/Froglok_chef L52/10500 hp
So if you look at this something should immediately jump out at you: The Ghoul Lord has only 7 levels on the Kobold Priest, but he has over three times as many hitpoints! In fact from L42 or so mobs gain about 1000HP per level. Basically in classic with player levels going up to 50, any mob around that level was considered a boss mob and got a corresponding hitpoint boost. What this means is that dps becomes more important, especially sustained melee dps. This is why when you ask people for their ideal group they will usually say something like War/Mnk/Rog/Rog/Cle/Enc where you have 4 pure melee and a charmed pet and you should hit 300+ dps on the parser.
The other problem druids have is that slow and complete heal both become ridiculous. At L40 when you get CH, your tank has 3k hp with buffs. At L60 it'll be 5k. So suddenly clerics are getting 8-10:1 heals while druids have 2.5:1 SH. Shamans get torpor (like a 6:1 heal) as well as slow which effectively makes their heals 4x better as the tank takes less damage. Druids get group regen, which isn't bad but isn't enough. The game would be much, much more balanced if Druids had Torpor instead of shamans imo.
Now, all that being said: you can play whatever class you want and be useful. There are plenty of horrible enchanters/monks/shamans and other "power classes" that aren't nearly as good as a good druid or ranger. The most important rule to "succeed" at 1999 is "be nice" (a rule I don't always manage to follow lol) and #2 is "play a lot".