Disclaimer: This is likely to be a controversial topic. In particular, I ask that everyone refrain from turning this into a "console peasants versus PC gaming master race" war. Furthermore, this topic is not intended to insult/bash consoles or console gamers - rather, it is intended for discussion of the decrease in quality PC games due to consoles.
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#1: Bad optimization, increased system specifications, and developer/publisher "double-dipping" with console ports.
In the 1990's, console games were almost completely separate from PC games. The main reason for this was that computers didn't have enough CPU/RAM resources to support shoddy console ports. As a result, PC games were optimized for PCs and console games were optimized for consoles. Everyone was happy, other than a few people who misconfigured their peripheral DMAs (direct memory access) and IRQs (interrupt queue priority) and crashed their computers.
These days (up until very recently with next-gen consoles), laptops/desktops greatly overtake consoles (Xbox 360, PS3) in terms of hardware specs. Developers start writing games optimized for consoles, then doing shoddy ports to Windows. The only reason why anyone even gets decent framerates out of the shoddy ports is that the average laptop/desktop was way ahead of an Xbox 360 or PS3. The design paradigm shifted to: Build a game, optimize for console, do a shoddy port to Windows, and ratchet up the system specs.
Now here's a counter-argument that I would expect: A good gaming desktop costs a decent amount of money and a gaming laptop costs even more. Consoles bring premium games to people who can't afford good desktops/laptops.
Time to debunk that. In the old days, PC games were optimized for PCs. Most PC games in the 1990s would run with mediocre framerate on any half-decent, mostly-outdated PC on minimal settings. Alternately you could get a premium desktop and turn the settings up. But even if you had a cheap computer, you weren't completely locked out - you just had to turn down the settings.
These days, if you have a lousy desktop/laptop, you are locked out of a lot of premium games because the optimizations aren't there anymore. Your choice is now between a premium desktop/laptop, or a console. This svcks because a console is just a crippled desktop.
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#2: Dumbed-Down UI and Controls
The developer "double-dipping" between consoles and PCs mentioned above has also caused a backstep for PC games. In the 1990's, for example, most FPS games, platformers, and flight combat games for Windows let your character have a large number weapons at a time (examples: Quake, Unreal Tournament, Terminal Velocity, Descent, Radix:Into the Void, Raptor:Call of the Shadows, Jazz Jackrabbit 2). You could pick weapons with the numeric keys on the keyboard.
These days, a lot of premium games are actually ported from consoles, and the new standard in FPS games is that your character can only have 2 weapons (because controllers have no arrays of number keys). Controls are limited and a lot of maneuvers use button combos, effectively dumbing down the keyboard and mouse into a controller mockup.
But wait - there's more! A lot of console ports were never designed to support a mouse, resulting in glitchy/lousy/buggy support for mouse controls. Anyone here played Skyrim on the PC with no UI enhancements or overhauls? you will know what I mean. In extreme cases, some console ports skip any semblance of a keyboard mouse interface and force you to scroll through 1-dimensional menus.
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#3: Content and update discrepancies
This mostly applies to online games. Console game services such as PlayStation Network or Xbox Live may require a lengthy certification/approval for updates. If a game with both console and PC versions is updated frequently, this means that either the PC version has to wait for the console versions to get approved, or they have to run different/incompatible builds, with the console version lagging behind.
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#4: The demise of classic premium games built for PCs
These days, money is everything for publishers and developers. If you design a game exclusively for the PC, you are automatically cutting yourself out of a large piece of the market. This is why console port "double-dipping" is so popular - maximizes profit at minimal cost.
This is good news for stockholders and publishers. This is also good for computer hardware manufacturers due to the ever-increasing system spec requirements. Guess who's paying for it?
Old-time PC gamers who remember the good old days in the 1990's, who understand what makes a quality PC game and what breaks a lousy PC game.
Even every other PC gamer is paying for it in the cost of mandatory computer upgrades. No upgrade = game won't run.
I rarely search for new computer games anymore from the mainstream video game industry because most of them made these days are so lousy. Recently, I've been mostly browsing GOG for anything new that remotely reminds me of the 1990's.