A comment from Bay's site re character design is also appropriate here as well, I reckon - to do with 'getting into the mind' of the character or something - what sort of person, what would they do, how would they react to a certain situation etc?
Applied to mechanical stuff in general could be- (no particular order)
What's the purpose of the item? What's it supposed to do? What would you like it to be able to do?
How does (would) it move? Are pivots in the right place - do they have the correct number of degrees of freedom - do the pivots/bearings look strong enough to take the loading suggested by the bulk/mass of the rest of the model? Will it balance ok - or are additional features required to assist - What's the likely sequence of actions required to get it to move? Tracks can look good sometimes.
Do pivots require / need full 360 deg travel - are constraints to be made obvious (imagine knees without 'stops')
What's it made of? - rough, smooth (combination?) shiny/matt etc - in the real world precision engineering costs money and is only normally used where necessary.
Pressed sheet metal panels often have 'raised / embossed' areas because these add stiffness with no extra weight penalty - such areas can be of 'interesting' design / layout.
Can it actually be made or is the creation incapable of being constructed /assembled?
Do the individual elements of the design 'hang together' to form an integrated approach to the subject matter, or do some things 'grate / not seem quite right?'
If to be controlled/operated by a person/figure/alien (in the picture/scene) do the ergonomics of the situation work? - or does the placement of the levers/controls etc require the presence of someone/thing totally different in shape and size?
Do these operating controls need some external (and visible) means of connection to the functions they're supposed to control?
Don't forget that pneumatic / hydraulic systems need supply (return) hoses / pipes - hydraulic rams without these will look somewhat strange imo.
External indications of internal workings - exhaust/intake ducts/pipes etc - sensors, rangefinders / radar etc comms aerials can sometimes enhance things.
Likely working/operating environment - what sort of grip required - does it need to be amphibious (or fly, even) - are alternative systems required - stowage pods or suchlike might give scope for a model in the act of extending / removing gear or equipment from pods with doors open etc?
Does it need windows/portholes/periscopes/ binocular rangefinders etc (like tanks/APC's etc)
Does it need suspension struts / dampers /springs or similar - some mechanisms require feedback linkages (and associated sensors) to work (also look complex) others might well use parallelogram type systems (like on some bus/coach windscreen wipers)
Lights - does it need them? Type /colour / number / quantity etc
If it's something that moves, bulk/mass/weight is often a problem - so lightening holes/ webs are often used (and look cool) -
Weapons/armour - do they have the necessary degrees of freedom to be able to work ok - range / arc of fire - if pivoted, are they in a logical position - cantilevered (unbalanced) gear working from an end pivot will be more difficult to control and require more power than balanced stuff - ammo drums/containers/magazines etc - spent ammo chutes?
Are the attachment points able / capable of using alternative systems - does the whole approach have to be symetrical or can there be different systems / weapons in each hand / claw / grab?
Don't let your creative juices become diluted with what's on offer around and about - unless you're modelling something specific that requires research, that is! - keep your eyes open and try to figure out (if you don't already) why bits of machinery around you are made in the way they are.
Do a sketch first? - no matter how rough - if anything like mine they're always good for a laugh several years downstream - in the past I made the mistake of doing mine on loose paper (and lost same) - now I use hard-backed sketch books.
The problem with putting lots of detail on something (mechanical) - from my point of view, is that it can invite very critical appraisal of the basics involved.
Keep the level of detail / complexity consistent - otherwise you're inviting comparisons to be made.
What's it do?
Why is it made like this?
Why does it look like this?
Extra features cost time / money / resources in real life - if not essential, they're left off.
Etc etc
If it's going to be a long(er) term project then (imo) you've got to like the subject matter for some reason - whatever it might be - since I don't know your interests the following may not be relevant/appropriate :)
Old motorbikes - pre 1930s or so, when everything's exposed and no covers etc.
Early steam locos - often very interesting approaches taken to overcome the age-old compromise of getting moving elements stong enough but as light as possible.
A 747 engine - minus cowling/covers, of course - if you really wanted to 'push the boat out' - a partly sectioned approach?
Machinery - which has exposed hydraulic/pneumatic cylinders and pipework runs - a vast choice ranging from any sort of earth moving (construction) equipment to cigarette production machinery - like that made by Molins.
Bicycle - Making a 'decent' one of these, without taking all the usual shortcuts, is not easy imo - having relatively complex (small) details at the ends of long, thin structures creates all manner of issues. Modern bikes with full suspension/disk brakes etc offer a lot of possibilities.
A 'garage' scene - A car of your choice - in a workshop/garage, with the bonnet(hood) opened, exposing the engine, maybe jacked up and a wheel (or 2) off showing the drivetrain.
Mechanical clock - The sort of thing you get inside a glass case - complete with the correct number (and shape) of teeth on relevant cogs.
Musical Instuments - Not a 'penny whistle', but something like a sax - or even the innards of a grand piano.
Cameras - Looking at one of my own (old Canon A1) - there's a lot of scope (for a detailed model) with all the odds and ends on the thing. More 'pro' type video cameras / camcorders offer even more scope.
Domestic appliances - like a washing machine, with a 'transparent' sheet metal outer case/shell. (There was a very interesting TV ad for the Dyson cleaner in the uk, using cg)