I've only played Dark Souls 1 and 2, neither of the Playstation exclusive one because I lack a Playstation and don't care about enough other games exclusive to the console to go buy one. The thing I like best about them is the combat. I haven't really enjoyed most action combat games I've played because they tend to boil down to just mashing through enemies like they're nothing or just spamming your best combo to destroy everything. I like the fact that in Dark Souls you can't just flail and spam against most enemies because that'll get you murdered. You have to play it smart even against normal enemies because they can absolutely kill you if you're not paying attention, and that interests me far more than "berserker rush your way through the level to get to the boss that might actually make for an interesting fight." Murdering monsters by the dozen without breaking a sweat just doesn't feel all that fun for me in these sorts of action games, which of course isn't an issue in Dark Souls.
As for the trailer, definitely gonna get and play Dark Souls 3. I'm hoping the fact that most of the bosses they showed were rehashes of things we've seen before (like that giant knight guy that crawls around on all fours that's clearly using the Sif model/AI, or the bigger guy that looks like it'll be similar to the Old Iron King) is just because they wanted to keep new things secret until later. Still gonna play it either way, because I like the game enough to play through the same boss type many times and can still have fun with it, but my one major complaint about Dark Souls 2 was the number of bosses that were just reskins of bosses from the first game.