This is a topic that regularly brings me great anger. Why are so many multiplatform games so poorly optimized for the PC? Yes, there are a few decent multiplatform engines (Unreal 3, MT Framework). Regardless, I fail to understand how games that run smoothly on a console released in 2005 require mid-range PCs (~$800) built in 2009. I'm looking at you, Assassin's Creed I/II and Frostbite Engine 2 (Bad Company 2).
I find this extremely offensive:
Bad Company 2's recommended configuration for the PC is a Core 2 Quad and a Radeon 4870. The cost of those 2 items alone is greater than the cost of a new Xbox 360. What? The game runs on a PowerPC chip designed in 2005 with a miserably weak GPU.
Developers are infuriatingly lazy with PC optimizations.

I don't have either of these things (okay, I have a 9800 GT but I don't think it costs that much) and Bad Company 2 runs amazingly well on my PC.

At what resolution and at what settings?
I've done some testing with the BC2 beta and I am not impressed with the engine's performance. It looks worse than Crysis yet requires better hardware.

The PC has always been that way, its the haven for laziness. On the C64 and amiga (and on modern consoles), games were coded and recoded until they worked properly, on the PC they just added the next CPU step, GPU or memory amount to the minimum requirements and ignored the problem.

Don't forget all the different hardware PC's have. It's practically impossible to make a game that works perfect for every PC.

1920 x 1080, with medium settings.

I'm really not sure that's as much of an issue these days, the variation in hardware might affect performance but it shouldn't affect the ability to run a game.
After all, there are two main CPU producers, two main graphics card producers, one main operating system producer... lots of variation within these groups, but you would assume some level of standardisation.
But I'm not that much of a tech, so perhaps I'm wrong.

You are right. Thanks to Direct3D (especially since D3D 10 and more uniform hardware specifications), the "variety of hardware" excuse is garbage.

Then how do you explain complaints about crashes and various other problems that always happens with a new game? There are always a percentage of gamers that have problems.
It's not unusual that people with the exact same hardware have varied experiences. For some it works and not for others. Usually these problems have to do with various settings or drivers.
PC is not a platform with a uniform standard and that's the main problem. Especially with new game engines.

One problem is that games get rushed out the door in order to capitalize on the hype that was built up before the game's release. The developer needs the time that the publisher won't give because they don't have the money from the game which is incomplete.
Another problem is the fact that it is ridiculously easy to patch a game after release unlike, say, back in 1995 when games could still be expected to come on floppy disks and the internet was not as ubiquitous as it is nowadays.
edited for clarity.

*Cough* Bioshock 2 *Cough*
Oh, i'm sorry, what?

Most games that use Unreal Engine 3 run really well on my computer. BioShock 2 doesn't.

Seriously, probably half or more of those cases are just dumbassery. :P
For few or no issues, a PC requires quite a bit of care and precaution. I don't think most people are like that.

Patch didn't help?

It just fixed the screen issue. Or at least I think it did. People don't seem to be complaining about it any more.