You need to know 2 things about your MEP consultant's scope of work:

• Enough about the sizes and layout of their services to determine when things are/will be in conflict because never in the history of the work has one engineer bothered to check with another (even when it's literally the same person doing both tasks) if their shit is in the way of another's shit.
• Enough about what they do so that you can call them out on their bullshit when they say that something can't be done.

General system operations and requirements. If you're calculating loads (other than a back-of-napkin types for magnitudes), you've gone too far. You should know how a particular system would impact other systems and general design. For example: Do you have a slab cut for a duct? What are the dimensions? Does it require extra steel bracing? Would it behoove you to locate ducts and risers in the same chase? Can you mix water/duct or electrical/duct or whatever? Check code. 

It also helps to have an idea of system cost (again, order of magnitude, not necessarily the exact number). 

Also have a decent understanding of the deliverables to the client and to the AHJ for each trade. You might think you just need a PL set, but did the site connect go out? Backflow? Fire Department requirements? Being familiar with what other sets need to be produced will help firm up your ability to coordinate across trades.

Ducts go where ducts go. I have been informed by a succession of mechanical engineers that this is a fundamental truth of existence which may not be challenged.

I am aware I am late to the discussion, but Mechanical Engineer / BIM Manager here. Although I do get the gist of the messages here and many do have merit based on the typical MEP designer/engineer, this is not always the case. For example, I personally have never just copied layouts from previous projects and always have discussions with the Architect upfront to suggest multiple layouts and strategies that try to minimize the impact to the aesthetics. This is not always possible as there are a LOT of utilities that have to be considered - not just the ductwork and mechanical equipment. I also have horror stories from Architect's that couldn't do simple math when the total required duct free area required exceeded the available 9" above ceiling, so it is a reciprocal thing where some people just aren't good at their profession.

The main issue I run into currently is there is little to no interstitial space due to the higher decorative ceilings (and framing), and the structure frequently prevents most if not all duct routing solutions because they are designing at the same time in most Revit projects (they should be almost done prior to MEP coming onboard). Also, the more you raise the ceilings and include massive curtain walls and windows, the higher the required loads and airflows become. This of course translates to larger ducts and equipment.

I would be interested in having a constructive discussion with Architects and I.D. designers to explore the driving factors on the design decisions and potential implications from both sides, and how to best resolve conflicts to get the best of both worlds.

Any takers?

I'm still here. I wouldn't call it a service,  I'm just interested in learning what the design constraints are from a different perspective and how people deal with them.

Unless your MEP engineer is developing drawings for very simple and conventional systems, you need to be careful and do your homework to make sure they are doing a thorough and correct job. I am working on a large residential project utilizing a lot of hydronic systems and our first mep engineer was completely screwing it up and leaving a critical amount of scope undeveloped and undocumented.  I had no idea the drawings were so problematic until we started talking to other MEP firms and showed them their drawings.

It’s kind of a no brainer that architects need to conceptually consider how heating and cooling is administered and placed in their designs. But there is another level of attention to the MEP documentation completion and system design functionality that is very important that we don’t have easy insight into without having more experience. The final thing is having the attention to detail as a designer in terms of adjusting the location and size of various mep elements (registers, grilles, drains, devices etc) in the space.