Summary: It may be more compatible with Windows 7/8/8.1 than HL1 normally is, but aside from compatibility and better lighting, there's not much to make this game worth your money.
First off, nothing has changed from HL1's multiplayer mode. The maps may be running on Source, but there's still no props or anything-- it's still all just the map geometry. More than that, you have to deal with the regular HL1 models without the HD pack, unless you feel the need to go through HL1 with Photoshop and do it yourself via modding (which will most likely not work on some servers).
Here's the real reason you want to stay away: 1) the GoldSrc HL1 has more players (source is pretty much empty, HL1 has a few players), and 2) this game is an EXTRA price. That's right. It's not like "oh, they just separated it because it's a big file" as Red Orchestra 2 and the CoD games do it. No, you have to BUY the singleplayer for HL:Source (a broken mess) for $10, then HL Deathmatch Source is a standalone for you to slap down more money for. Why not just buy HL1, with Deathmatch mode included?
Aside from what's above, what's advertised in HL Source isn't really that great. The ragdoll physics are present, yes... however, they're bugged. VERY bugged. They lock up as soon as you die and you fall sideways in a stupid pose, whether you're posed to run in place on the ground, you're failing at parkour by getting your arm phased into a guardrail, or you're doing a handstand... sideways. Other than ragdoll physics, the lighting looks better and the skybox is now 3D rather than a few jpg's of a New Mexico backdrop, but other than that this game has no real improvements.
Final verdict: It's a waste of money. Just buy the old HL1, which will include this mode AND the singleplayer for $10 total. It will also have HD models and the old death-animations rather than the failed ragdolls you see here. It also has more players and better servers (both vanilla and modservers as opposed to a few vanilla).
It's not worth your cash, just add the original Half-Life (not HL Source) to your cart and move along.
EDIT: Take a look at these next gen ragdoll physics!http://steamcommunity.com/sharedfiles/filedetails/?id=345473153