As for what chapter allows for what possibilities in and of itself (since it confuses lots of people,) I'll give it a shot here.
If you're buying only one of the three chapters ...
All three chapters give you access to some skills for the core professions: Warrior, Monk, Ranger, Elementalist, Mesmer, and Necromancer. You can create a character (PvE/PvP or PvP-only) with any of the "core professions" as primary or secondary in any chapter. Any one chapter will give you the ability to get all the skills considered "core skills" (regular and elite skills available to core characters in any chapter) and some more skills for "core professions" considered chapter-specific.
Ritualist and Assassin characters may only be created with Factions (chapter 2.) Factions has the most skills available for the Rt and A professions (I think.) Factions has the core skills, of course, and some new chapter-specific skills for core professions.
Dervish and Paragon characters may only be created with Nightfall (chapter 3.) NF has the only skills currently available for D and P professions. There are some chapter-specific skills available for the Rt and A in NF. Like Factions, there are the core skills and some new chapter-specific skills for the core professions. Don't buy *only* NF if you want to play with Rt and A, though (see above.)
Prophecies (chapter 1) was first. So there are no Rt, A, D, or P skills available there.
Factions was second, so there are no D or P skills available there.
Nightfall is third. You'd almost think there wouldn't be Rt or A skills available there, but there are some. Think of it as ANet doesn't update old chapters for new professions, but they'd have been hurting people playing Rt and A with both Factions and NF, so the professions from a specific older chapter get some new tricks in newer chapters, at least so far.
I suspect the inclusion of skills for the non-core and non-NF Rt and A professions in Nightfall is the source of much confusion to people unfamiliar with the game.
Here's the best way I can think to give guidelines for choosing only one chapter.
- If you want to play with the core professions and only get one chapter, you might consider reading the profession skill description lists on wiki, starting here. They are links to nicely laid-out tables of each professions skills with indications of what's core and what's chapter-specific. Bear in mind that wiki, like guru, is a fansite built and maintained by fans, not ANet or NCSoft. Folks are pretty good at keeping it current, but it might not always match the exact current state of the game.
- If you want to play with the newer, chapter-specific professions, get that chapter (Factions for Rt and A, Nightfall for D and P.)
- If you want to get the most "free" skills from quests in PvE for core characters, start with Prophecies. Factions and Nightfall each give you a few free skills for new characters in the beginning areas, but Prophecies has the most skill quests available by far.
- If you want to get only one chapter and have the most likelihood of making/finding builds to compete with in PvP, reading the PvP forums is a good place to start. The current general opinion *seems to be* that NF has the most powerful skills available, at least for some professions. Such things are very subject to change, however, as ANet makes "skill balance updates" (nerfs and buffs) at least once a season. Disclaimer - I'm far from a PvP expert, so this may not be right. This is my best educated guess from playing all three chapters, chatting with friends, and browsing forums.
Sorry for the long post. I hope it helps. And you think that's bad? It'd be even more confusing if I tried an answer to "which 2 of the 3 chapters should I buy?" questions!
Oh, and I think this is all correct, but if I've made mistakes, please do PM me and I'll try to update this.
GL in your decisions. I hope y'all find the game as enjoyable as I do!
Luny