What do I need to look for in a Blu-ray player?

Given you have defined a budget that only allows a lower end player your best bet is to look for one that comes from a name brand manufacturer, meets your budget limits and has a good warranty.

All Blu-ray players will play Blu-ray disks, DVDs, and CDs, can be set to various output resolutions, and have various output ports to allow connection to almost any display unit. As you pay more you gain features like better DVD scaling and multi-channel analog sound capability. The first is highly desirable for those who want to continue to play their DVD collection. The latter is best for those with non-HDMI sound systems (but essentially useless to most consumers). Players may offer additional features like SACD playback, network streaming, external media server link capability, support for esoteric formats, USB ports for photo/MP3/WMA playback, wireless connectivity, etc. These features are likely not of much use to you for a bedroom player (Note that there is no point in a Blu-ray player at all if you are not going to connect to a 37"+ HDTV ... stick with an upscaling DVD player).

Assuming you meet the HDTV requirement (or plan to soon), and given your $100 budget, I'd ignorte all the extra features and look for a discontinued or used 'better' Version 1.1 player (not "Blu-ray Live"), i.e. an older discontinued model that originally sold for $200 or so. It'll work fine for Blu-ray and DVDs and any additional features it happens to have are gravy. Read reviews like those at the link for pros and cons of newer machines.

I recently bought an LG BD560 for $129 and it works quite well (even load faster than most). Not the best DVD upscaler (it costs $$ for better video processors), but certainly good enough for casual use.

You can't go wrong buying a Panasonic BD-65, they are the most reliable player in the market today. I still use my Panasonic BD-10 (the first generation blu ray player made) and have had no problems at all over the years. Go online to Home Theater Magazine and read the reviews on blu ray disc players. Hope this will help you out.