There's two major factors that make this sort of army dangerous: the Medivacs, and the Marauders.
The reason why there'd be Medivacs is to make sure the army stays at or near full health when the Marines and Marauders are stimmed. The Marauders are there mostly for grinding through armoured ground forces, which is why the Marines are in there.
Of course, the real influencing factor here are the Medivacs (which are better off being just aerial transports than mobile healer units). As either race, I'd highly recommend having both a capable ground force to combat his M&M (Marines & Marauders) and an air force meant for taking the Medivacs out of action.
Maybe someone should write up a sort of Guide to SC2 melee? Thinking about it, it'll really help out those that are still struggling with it (whether it's the economy, build orders, expanding, or the army).
"I figure language is a poor enough a means of communication as it is so we have to use all the words we've got. Besides, there are damned few words that everybody understands"
~Henry Drummond, Lawyer from Inherit the Wind
DTs are possible if the terran doesn't pay attention. Otherwise immortals if it's late game and early game stalkers and zealots can be pretty good at handling them if you focus fire the marauders and medivacs down.
Just pray he doesn't get Ravens if you decide to use Dark Templar. Then there's the High Templar's Psionic Storm, which damages everything in it's area effect (use it just below where the Medivacs fly).
Or grab both if you're capable at multitasking,
Of course you can also use Sentries and their Hallucinations to keep his units distracted (unless he's smart enough to notice or grabs Ravens) while your other units can go to town with his Medivacs and Marauders.
"I figure language is a poor enough a means of communication as it is so we have to use all the words we've got. Besides, there are damned few words that everybody understands"
~Henry Drummond, Lawyer from Inherit the Wind
I'd rather go the hard way: Zealots for cannon fodder, some Stalkers, Sentries to launch force fields and piss him off, and some Colossi too. Not necessarily this order, but it's the best.
How I became a troll in a single post (And you can too!!! ): link
Mostly they don't get Ravens, I haven't experienced a single MM&M that isn't countered by either Dark Templars or Void Rays as Protoss, or Brood Lords and any AA for Zerg.