World-of-warcraft – Are World of Warcraft quests linear

This question gets a bit tricky to answer... Starting from scratch, if you buy just vanilla WoW, you can experience the original world as it has been destroyed by the Cataclysm expansion. You can level up to level 60, but cannot access professions, areas, races, or other such things from any of the other expansions.

In order to access Outland, jewelcrafting profession, and Draenei? / Blood Elf races and starting areas, and level 61-70, you will need vanilla WoW + the Burning Crusade expansion.

In order to access Northrend, Inscription profession, and Death Knight class, and level 71-80, you will need vanilla Wow, Burning Crusade expansion, and Wrath of the Lich King expansion. Oh, and a prior level 55+ character on your account to play as a Death Knight.

In order to access the misc Cataclysm level 80+ areas, Archeology secondary profession, Worgen / Goblin races, and level 81-85, you will need vanilla WoW, Burning Crusade expanion, Wrath of the Lich King expansion, and the Cataclysm expansion.

Those are the restrictions. You can play on whatever realm you want with whoever you wish. You just may not be able to reach all the areas they can.

You can trial every bit of it with the 10 day trial, though I don't quite know how they treat it when your time is up.

While a list of content is all well and good, the question asks: what was the 'end-game' content, and what's likely to be revamped? Maraudon was only briefly an end-game instance, and it was intended to plug a progression hole.

So, in order, this is where Blizzard intended level capped players to spend their time:

• In vanilla WoW, one of the most lucrative sources for loot was the Dire Maul tribute run. This involves visiting Dire Maul North, and avoiding all of the bosses except for the final one. In Cataclysm, Dire Maul was revamped, but I believe the tribute run was preserved.
• Timbermaw Hold was one of the primary rep grinds; it is mostly intact but considerably abbreviated in Cataclysm. It starts in Felwood's south; there is a Timbermaw NPC that will give you a kill quest to start the chain. It involves quite a bit of grinding on furbolgs in order to get bracelets that can be handed in for reputation. Previously, you had to travel through Timbermaw Hold to reach Winterspring, which involved either killing or being of sufficient reputation so you could flag yourself as non-hostile; thanks to Cataclysm changes, this is no longer required.
• The Thorium Brotherhood gave blacksmiths special recipes for reaching exalted with them. Like Timbermaw Hold, this rep grind has been greatly abbreviated in Cataclysm. You can get a foothold with them by visiting Thorium Point in Searing Gorge; to continue past revered, you need to visit their deep cover operative in Blackrock Depths, in the back room of the Grim Guzzler, and give him a handful of Dark Iron Residue.
• The Argent Dawn were one of the major rep grinds; this rep grind was partially removed in Wrath (chiefly Naxxramas attunement) and gutted in Cataclysm. It involved equipping an Argent Dawn Commission and killing undead, which would randomly drop scourgestones that could be turned in for reputation. You could also visit Stratholme and Scholomance and earn reputation for clearing the dungeon, and visit Light's Hope Chapel and pick up a crafting writ, which would give you a quest to collect end-game crafting goods. None of these methods remain in the game; revered with the Argent Dawn comes from following the Fiona's Caravan in Western Plaguelands all the way to Light's Hope Chapel, and there are repeatable quests at Light's Hope Chapel to raise your Argent Dawn reputation from there.
• The last remaining reputation grind from vanilla WoW are the Cenarion Circle in Silithus. There are three separate sources for rep here: clearing the Ruins of Ahn-Quiraj raid; completing a Field Duty (a proto-daily) quest; and killing Twilight cultists and summoning elementals. This has been nerfed, but is, like the zone itself, mostly intact from vanilla WoW.
• An NPC in Winterspring (for Alliance) or Un'goro Crater (for Horde) has a series of quests for you to earn a mount. The Un'goro Crater quests are a recent addition, but the Alliance NPC (the Wintersaber Trainers) is from vanilla WoW. This used to involve a rep grind; the reputation exists, but the mount is now awarded based on completing daily quests and exalted comes for free.
• The Darkmoon Faire will be revamped in 4.3; check it out now if you want to see it in its current (terrible) state.
• In Burning Crusade, several new factions were added, all tied to their own winged dungeon. A big part of the end-game was raising these factions to exalted. Special note should be given to Tempest Keep, which was intended as an end-game dungeon.
• The Mag'har and Kurenai will give you a talbuk mount in return for killing a massive amount of ogres in Nagrand. The ogres also drop Obsidium Warbeads, which can be given to either the Mag'har/Kurenai or the Consortium.
• Daily hubs were introduced in Burning Crusade; the first was the Skyguard, based in Blackwing Landing in Terrokar Forest. They have a few dailies (a bombing run and an escort quest that starts at one of four cages) and a quest chain that involves a fair bit of Cenarion Circle-style mob grinding, with a summonable end-boss that's mostly trivial for 85s.
• The second quest hub is Ogri'la. This used to require a tricky quest chain to complete, starting in Shattrath and involving several elites; this is no longer required, but the quest chain is fun. It's entirely daily-based, and ramps up as you reach new rep levels; you also unlock additional Skyguard quests here. Make sure you check out the Shartuul event; it requires a Darkrune to start, and is still challenging. (It is, however, easily bugged, like many BC events.)
• There's a quest chain that starts from an NPC in Netherwing Fields that will start the Netherwing dailies; you need Artisan Riding to progress beyond neutral and unlock the Netherwing daily hub. This was the most prestigious of BC content, and is still one of the most appealing rep grinds in the game.
• The Consortium's rep grind grew in importance through BC. There are a few sources of Consortium rep, but the most important ones are in Netherstorm. You can grind for Zaxxis Insignias and clear Mana-Tombs, but the more notable area is the Ethereum Staging Grounds. There are Ethereum Prisons here that can be opened with Ethereum Prison Keys, available as random drops from the enemies here, or from doing daily dungeon quests in Shattrath. Eventually, you'll be able to unlock an extra boss in Mana-Tombs.
• The Shattari Sun Offensive is the final daily hub in Burning Crusade; there's a teleport there from Shattrath. Blizzard enjoy revisiting this zone, so it's the most likely candidate to be repurposed in the future.
• Sporeggar is a minor daily hub that involves killing naga and collecting plants from the Underbog.
• There's an extra boss in Sethekk Halls that used to be part of the druid epic flight form quest. You no longer need a druid to summon him, and he occasionally drops the Raven Lord mount.
• In Wrath of the Lich King, end-game reputations were mostly daily quest-based. The quest hub at launch was the Sons of Hodir, unlocked via a quest chain in Storm Peaks, starting with They Took Our Men!
• The Kalu'ak have three settlements on Northrend's south coast, and give out a few dailies.
• Sholozar Basin has two duelling quest hubs, the Frenzyheart and the Oracles. To unlock them you need to complete the quest chain, starting with The Part-Time Hunter, and choose which faction to ally yourself with by sparing them during the last quest. You can repeat this battle to switch sides at any time. (Most players stuck with the Oracles, as they can potentially award a green proto-drake as a reward; if you intend to do both sides and get the achievement for doing so, start with the Frenzyheart.)
• The Agent Tournament, you've already run into.
• Players were also encouraged to raise reputation with neutral factions by running dungeons - as far as I'm aware, these tabards still work in level 85 dungeons. If you're brave, you could try soloing level 80 dungeons.