It's a whole new game

Imagine launching yourself down a water-slide. You slip and steady yourself, pick up speed and learn to lean into curves, you flow with the current, aero and hydro-dynamic... yet as you reach the bottom, instead of cannonballing into a gigantic warm pool, you slow, and slow, and slow, and get gradually tipped over the side onto a rubber mat. You're handed a towel and told that the rubber-mat chapter of sliding is meant to maintain the freshness and novelty of slide-play - that it is, in fact, an improvement on the concept, as shown in these carefully powerpointed focus-group studies.
Somewhere behind you, ecstatically screeching youngsters splash into a sunny pool.

1. Boom - and that's the game, now.

I haven't been in the mood for much lately. Don't wanna write, dun wanna read, dun wan' no new movies or games or nuthin' that might prove an unpleasant surprise. So instead I've opted for dependable unpleasantness:
Defense of the Ancients (possibly #2, it's kinda hard to tell)

But it turns out this was a good time to dive back into yon murky waters, since I get to catch the re-introduction of one of DotA's more controversial stabs at creativity, the Goblin Techies. Choosing this hero allows you to create invisible minefields powerful enough to remotely one-shot even the toughest enemies in a very satisfying pyroclastic illustration of the term "overkill" - very aggravating if you're on the receiving end. In itself, this is great. The techies function much more in the way heroes should in a team strategy game.... strategically. Predict the enemy's movements, plan your trap, catch an opportune moment when no-one's looking and set up some nasty surprises. Nothing so crass as direct confrontation.
That the techies should be so unusual within DotA's greater scheme is a perfect encapsulation of the game's failings. The techies are a good hero, a strategic hero. DotA is not a strategic game. It's a twitchfest in which macho strength and agility heroes (jocks) beat on intel-based nerds until they drop. Its basic draw is the chance to play an athletic, rich and invincible warrior beating a scrawny spellcaster's teeth in, in a very personal and in-your-face manner. The techies are a better basic concept... but they don't fit into the game plan. It's not about unfairness - DotA was built around imbalance - but things feel very differently with techies in the field: an entire playerbase suddenly has to start worrying about surveying large areas where the techies might be setting traps. People have to learn to think in terms of choke points, tactical retreats and planned offensives.

And that's a problem. Potentially useless, potentially devastating, the techies (one playable hero out of dozens) alter the basic function and flow of gameplay. Games are partly about developing a very specialized set of skills, and it's a perfectly reasonable expectation that further expansion of any particular game concept, its progression, should build on those skills. It's very clumsy design that which, after a few score matches played, suddenly allows a single game element to negate the basic ruleset - yet all too common. If DotA has some excuse for this (having been after all developed in the truest sense of the word by amateurs) this sort of random oncogenic end-game mutation has taken over much more expensive projects as well.

2. Lord of the now-powerless rings.

Take LotRO, for instance, a basic third-person online RPG. You choose your class, walk around, and use skills, from your basic firebolts to the even more basic fire-balls. These skills, to a great extent, are the gameplay and your identity. Your hero's skills (the arrows and fireballs you shoot, the sword you swing, the extent to which you can heal yourself and your companions) are your method of interacting with the game world. However, the Helm's Deep expansion presented players with a new type of content: "epic" battles. They were meant partly to incorporate level-scaling, to compensate somewhat for the ludicrous amount of time new players are forced to invest in order to catch up to and play with their top-level buddies, and thus are accessible from level ten. A level ten character in LotRO has access to only a handful of skills and no gear to speak of, so to even the playing field, an entirely new interface was overlaid on the existing one.

All of a sudden, after a hundred levels during which you've dutifully ground out your hard-earned array of magic missiles and axe-swings, you're faced with half-hour affairs during which, instead of fighting, your main task is playing whack-a-mole with siege ladders and constantly refreshing the same order on squads of soldiers. Were the entire game built around this, it might have turned out to be a much better experience than LotRO's simplistic hack'n'slash, but to someone who's spent months and years polishing those hack / slash skill icons in his character's taskbar, seeing them suddenly lying there lonely and snubbed can feel, quite rightly, like being cheated out of a pay-off.

3. I'll trade you one toy for your ten.

Some features are just too central to a game concept to be abandoned in the interest of novelty value. If in a third-person RPG the interface and the player's activity revolves around the skill buttons, in a First-Person-Shooter this focus is on your... shooter(s). Here the miscreant is once again Valve.
Half-Life has remained one of the most important reference points in computer games. It was a lovely piece of work all-in-all: atmospheric, action-packed, clever, well-organized and very satisfying. Its ending, among other things, was appropriately cathartic. Though you lose all your guns halfway through the story (and get thrown into a trash compactor, but that's a different matter) you gradually recover them all and then some, and by the time you reach the end boss you're packing enough heat to level a medium-sized city. The final fight had you let loose at a kaiju with whatever you wished, all your rifles, shotgun, rockets, gluon streams and alien hornet cannon in an appropriately climactic grand finale.
Half-Life 2, on the other hand, while an expertly orchestrated adventure in most respects, failed miserably in its final stretch. Your minutely, lovingly-assembled arsenal is taken from you in a cinematic and you spend the final moment of the game knocking some panels out of place with a nerf cannon. Whether or not this was anti-climactic in terms of storytelling is not the point. Games are interactive endeavors playing off of and manipulating the players' input. To suddenly place severe constraints on that input just at the culmination of the player's efforts can sour the whole experience.

4. It's all over but the gimmie.

Planescape: Torment, at least as noteworthy a project as Half-Life, was an RPG centered on the traditional routine of killing things and completing tasks in order to advance not only the story but one's character. Shortly before the end, however, you're handed a massive exp reward which in effect brings all players, regardless of their previous success, to the same level. Not only that, but you're separated from all the colorful characters' whose companionship you've cultivated throughout your journey of self-discovery and made to fight your way through the final stages alone. Anticlimactic? Not quite, in terms of storytelling - this sequence of events was fitting. Yet from a gameplay perspective, this last stage was all too forgettable. Your efforts invalidated by a hand-out of experience, your strategy negated by removal of your main tools, you find yourself wondering when the real game will start again.

5. Eturnity liess uhhed uv uss, ahnd beeheind.

One of the staples of the Civilization series has been the exodus from a climatically-destabilized Earth aboard a colony ship, an end-game rush lending some urgency to your empire-building. In the series' brilliant spin-off, Alpha Centauri, the concept was taken a bit further. Inspired by stories like the Pandora series, Planet is the eighth of seven factions, a reactive AI participant in gameplay whose power and level of activity ramp up to implacably apocalyptic levels toward the end. It's a whole new game, and as your improvements explode into fungal forests and swarms of locusts lay waste to your cities you can find attempted deicide taking up much more of your attention than exterminating other mere mortals like yourself.
However, in counterpoint to previous examples, Alpha Centauri does not restrict player agency in order to make its big finish seem new and exciting. The tools you use to hold off Planet's attacks and rebuild are the same military units and terraformers you've been building up all game. Your infrastructure depends on the same cities. You are still you, and your intent is still manifest in your approach to the endgame. One need not render existing game mechanics and previous player effort useless in order to introduce an end-game quirk. Novelty must be introduced along a continuum.

Have you drunk your fill?