What I never understand is how BioWare can make these iterations in the DLC and then completely discard them in the next game. I suppose it's up to the game development cycle, i.e. ME3's gameplay was already too far into development to be affected by LotSB, but I have to wonder if it's more because DLCs have much smaller scopes. BioWare just have more time to tighten up the few enemy encounters present in the DLC.
There might also be a greater obligation to innovate in a DLC. The devs have to come up with neat gameplay hooks within the existing systems rather than invent totally new gimmicks, so they stretch the base mechanics in cool ways. This obligation doesn't really exist in the base game once the combat mechanics are finalized, so developers have no incentive to constantly freshen up late game content.
If only there was a clean way to retroactively enhance the base game's level and enemy design with every release of a DLC.
You're right. The improvements that appear in DLCs don't consistently transition to the sequels. Maybe it's the development cycle schedule, as you mentioned. Sometimes, it could be because they're already reinventing the wheel, so the slight enhancements are no longer all that helpful. I'm not sure.
I'm very confident that they heard our comments about ME3 being easy, and took it as a challenge. I just hope they don't go "full BioWare" and go too far off into left field with their changes. Tuning is needed, not a total rebuild. BioWare has a history, almost a tradition, of taking player feedback way too seriously and with poor outcomes. We have heard that the gameplay is reminiscent of ME3, so likely they realize that the foundation is solid.