Hello and welcome to Dakka!
I too played Warmachine before I got into 40k. It's quite different, more simplistic, less tactical and more predictable.
You can't really protect anything, because weapon ranges are so enormous and your opponent may deep strike in units anywhere that's more than 9" away from you and fire away. Get used to losing a ton of models turn one if you go second. On the other hand, there's no warcaster that loses you the game if it gets killed.
I'm not that knowledgable about most 40k factions, I really only know anything about the ones me and my friends play (Craftworlds, Drukhari, Space Marines, Harlequins and Astra Militarum), and none of those have any debuffs to speak of. The only "attack vector denial" methods I know of that are doable are "bubble wrapping" against deep strikes and charges, positioning to protect your characters (who can only be shot at if they're the closest enemy unit) and LOS blocking. It's very basic compared to WMH.
If there are any debuffs to speak of, they're likely in the form of psychic powers, spells that certain models can cast. Craftworld Eldar have Doom, for instance, which provides rerolls on failed wound rolls against a target unit. That's akin to an ARM debuff in WMH.
There are people out there who know way more about the game than I do, who can perhaps help you more with choosing a faction you're going to enjoy. My primary advice is to choose something you like visually, it's easier to learn to like playing an army if you think it looks cool.