What makes a good Blu-ray player?

You can get a decent Blu-ray player for less than $100 these days, but many models cost much more. Are they truly better? What do they offer to justify the higher price?
And most importantly, are they worth the extra money?
The Basics
Obviously, all Blu-ray players are going to play Blu-ray discs. All will also play DVDs and CDs. Most will play images and perhaps movies off USB drives. All have HDMI outputs. Most new models will not have component outputs, thanks to the closure of the analog hole. They should all be able to send your receiver Dolby TrueHD, DTS-HD Master Audio, and the other new audio formats.
Streaming/Network
Netflix is awesome, and if you haven't experienced the greatness of streaming content, it's worth getting a new Blu-ray player just for this. This feature is available even on models less than $100, but the way it is implemented varies greatly.
While pretty much all players that have Internet-streaming capabilities will have Netflix, there are a few other streaming providers worth looking for. VUDU has access to more recent movies and high-resolution video. Amazon Video on Demand is, in my opinion, a necessity, as you can rent/buy current TV shows and movies.
Pandora or Slacker both offer nearly unlimited streaming music, and are quite addictive.
Most new Blu-ray players will now output Netflix content at 1080p, though the picture quality isn't anywhere near what you'd get from a Blu-ray disc.
As I mentioned in my "Should you upgrade" article, I've found that the picture quality from services like Netflix looks better coming from a Blu-ray player than it does from dedicated digital media receivers like Apple TV or Roku.
Most players with streaming capability are also DLNA-Certified, as in they can stream music from your computer. The implementation and functionality of this varies greatly. If this sounds like something you'd like to do, it's worth reading reviews to see how well the feature functions on each device.
Related articles:
What makes a good HDTV?
What is upconverting?
How to get free HDTV
Wireless
If you can't easily run an Ethernet cable to your Blu-ray player, it's worth checking out a player that has built-in Wi-Fi. Most players have the option of adding a wireless "dongle," for extra cost, but this rarely saves money over the players with wireless built in.
It's also worth checking out Matt Moskovciak's great article on Wi-Fi alternatives for your home theater.
Upconverting/Scaling
One difference between Blu-ray players is how well they upconvert, or scale, standard-definition material so it can be displayed on HDTVs. Upconverting affects not only DVDs, but also standard definition content from Internet sources like Netflix. It's worth noting that upconverting performance has no effect on Blu-ray disc playback.

The difference between a real HD image (top) and a standard definition image (bottom).
(Credit: Geoffrey Morrison)
Generally speaking, if a manufacturer calls out a scaler by name, its often pretty good. Companies like Marvell, Silicon Optix, and Anchor Bay all specialize in scaling, and I've yet to see a product featuring any of them that didn't upconvert really well.
But that's not to say that "in-house" designs can't be excellent. In my testing, Panasonic and Toshiba have great scaling, and they're not alone in that regard.
In the end, the difference between a great scaler and a mediocre scaler isn't going to be night and day. One will be "really good" while the other will just be "good." At least, when it comes to performance from Blu-ray players.
For more on how upconverting/scaling works, and what separates the good from the really good, check out my recent article on the topic.
3D
If you have a 3D TV, or are planning on getting one, you're going to need a player that can output 3D from 3D discs. There isn't a big price premium for this feature, and most future TVs will be 3D-capable, so it might be worth spending a bit more to get a 3D-capable player. Not a huge deal either way, though.
DVD-Audio/SACD
For the audiophile, the ability to play back the pretty-much-deceased DVD-Audio and SACD formats is a boon. I still have a decent collection of these high-resolution discs, and play them on occasion. These players often also feature high-end digital-to-analog converters (DACs) that can make even CDs sound better. The only way this feature is worthwhile is if you plan on hooking the player up to your receiver with analog audio cables. A Blu-ray player's DACs are bypassed if you use HDMI.
Less important/situationally important
There a few more features that are often marketed loudly, but are of limited use for most people. Some players have multiple HDMI outputs. Most often this is for people who have an older receiver that won't pass a 3D signal. One of the player's HDMI outputs gets sent to the TV directly, and the other goes to the receiver for the audio playback. Another option for this is using the Blu-ray player's optical or coax digital audio output, but you won't get Blu-ray's new audio formats, as those are only transmitted digitally over HDMI.
You could also, in theory, plug two TVs into the same Blu-ray player. Keep in mind, though, that some secondary outputs don't have video, or don't use the same scaler as the main output. In other words, you may not get the same performance out of both outputs. Don't let the retailer sell you expensive HDMI cables, though.They're pretty much all the same.
Many companies boast the "ultraquick" boot-up times of their players. Nearly all current Blu-ray players boot up quickly. The days of waiting over a minute just to see a loading screen are largely behind us. This shouldn't factor into your buying decision anymore.There is still some difference in how quickly different players load certain discs, but we're not talking minutes--many seconds, maybe.
It's also worth noting that if a player has a separate "fast boot" option, this will cause the player to draw more electricity when it's off.
Some high-end players will have multichannel analog outputs. Usually these are models that have special DACs or play DVD-Audio/SACD (though not always). If you have an older receiver, a multichannel analog output will let you hear the new audio formats in all their glory. Of course, that's presuming your receiver has a multichannel analog input.
One of my favorite niche features is the capability to output DVD content at 1080p/24. Certain TVs can adjust their framerates and/or play 24p material at a multiple of 24 (48, 96, some 120/240Hz TVs). This makes for a smoother movie experience. All Blu-ray movies are 1080p/24 natively--and nearly all players can output them as such--but only certain players can do the same with DVD (others output them at 1080p/60). I love this feature, but most people don't need it. There's also no simple way to tell which players will have this feature. Some big-money Blu-ray players I've reviewed can't do this, and some budget models I've reviewed can.
Conclusion
Speaking in generalities, I'd say: get the cheapest player you can that has the Internet-streaming services you think you'd use. Everyone should start there. Then add in the features you think you'd need, like wireless, 3D, or DVD-Audio/SACD playback.
Blu-ray really is worth it, and will make your HDTV look better than it ever has. The streaming services are a constant source of entertainment, and are almost equally worth it. Many excellent new Blu-ray players are a steal at between $100-$200. The more expensive models, in my testing, can perform a little better, but are definitely aimed at the enthusiast. You don't need to spend $1,000 to get an amazing Blu-ray experience. For a start, here's CNET's page on the best Blu-ray players.