I still dont understand why coolant is on every single mech. Coolant pods were an actual piece of equipment like BHP or GECM. Unless we're just going to hand-wave coolant being on everything away.
It's irreparable harm, that MW2 done to us. 
Fixt.
Wrong again. I'm almost definitely sure that MW2 had coolant. Not planning on spending several days trying to get it to work so I can check, though. 
As it was said already, MW2 had no coolant pods at all, neither free nor mountable alike. MW3's coolant was harmless to overall heat mechanics. It's MW$ err... MW4 that is a culprit here, with it's huge amount of free coolant.
Oh, then I'm probably thinking of the bug that I noticed after replaying it on a vista machine for the first time in ~7 years. Evidently, since the first P4 processors it's been a problem... some element of the heat calculation is based on processor speed or something. In short, it shuts you down when you redline but then does nothing. PULSE LASERS HOOOOOOOOOOOOOOO!
Yes, MW2 had seemingly forgiving heat mechanics, but it still had reactor explosion on ooverheat, that is instant death. Besides, MW2:GBL and MW2:MERCs was much more serious on the whole heat issue.
The real deal, though was heat management in MW3. A single spike of heat that goes over the top and
boom, you're dead. One Nova's alphastrike could make your reactor go critical, you wouldn't even have a chance to use your coolant in that situation. It was really harsh but it made us watch the heat closely and not overdo energy boating. 4xERPPC NCat? Ha! It would be a deathtrap if used as per MWLL. That's the heat system I'f like to see in MWLL. Hell, we could even leave MW3-styled two-shot free collant. It's not that it had real impact on heat management in MW3 anyway.