So we're comparing the huge swath of capabilities of vehicles to the huge swath of capabilities of mechs. What's the difference? The inherent difference between vehicles and mechs (afaik) - mechs are intended to make use of the structural benefit of the humanoid build, only on a much larger, customizable scale. So comparisons only work where what's being compared demonstrates that fundamental difference between a 'mech' and a 'vehicle'. Can one with absolute certainty say that vehicles are better than mechs? Absolutely not.

: Yes, but what's it going to do that a Warthog can't?

Anything a vehicle with two legs can do that a vehicle with two wheels can't. Whether housing a live human pilot or not. Anything the builders decide to add to the mech that allow the mech to do something the vehicle can't. You want specifics? Take your machine to a robot tournament or something :P That's what they live for - my construction is better than yours at what it's meant to do.

: Except if it's a tank analogue, there's still a number of factors working
: against it.
: -You've got a taller profile. While the cross-sectional area presented to the
: enemy might not be much larger than a Scorpion, you can be spotted from
: further away, and are probably more noticeable in general.

If that's a weakness, hopefully the builders would know this, and either alter its intended purpose or optimal strategy, or build it stronger to withstand the added risk of attack. It's not cut and dry.

: -For a tank and a mech of the same weight/volume, the mech has more surface
: area, and will require less armor coverage or thinner armor to stay in the
: same weight range.

You're limiting what the builders can build. "Tank" is a vague term. Wide variety of vehicular tanks, and wide variety of potential mech tanks. You're attempting to compare a 'better' vehicle to willingly handicapped mech. Sure the comparison might be valid for a specific pairing, but every construct has its weaknesses. Part of the development process is finding weaknesses and either accepting them, changing use strategy, working around them, or fixing them; whether building a vehicle or mech. Your question(s) would hopefully be addressed during the planning/construction process.

: -Guns. A tank is well-designed for transferring recoil to the ground. Mechs
: can brace themselves if they have heavy cannons, but Scorpions don't have
: to. And yes, you could mount lower-recoil weapons like autocannons,
: missiles, mortars, etc, but what is a mech going to do with them that a
: conventional vehicle can't?

Again, it depends on the strategy employed by the designers. If there is a weakness to its structure, then don't build it to make use of something hindered by that weakness. Practically speaking what does that look like? I don't know! Because mechs are highly customizable :P (as are, theoretically, non-mech vehicles)

: A vehicle is being designed with a specific purpose. Alright. What design
: features promote mechs over vehicles for that purpose?

See first paragraph. You have the inherent difference between a 'mech' and a 'vehicle' (for the sake of this discussion). So, if you're building a mech, how can you take advantage of that difference to the purpose for which you're building the mech? I'm not a mech builder. I don't know. I just know that "vehicles are always better than mechs" is incredibly short-sighted and flawed.

: The sole instance (That this thread has actually convinced me of) where a
: mech would be more useful than a more conventional vehicle is in heavily
: forested woodland where the forest canopy would impede close air support.

You seem to focus on visible foot print being an inherent and unavoidable weakness of mechs which across the board makes them worse. Can't agree with that

: I'll pass.

To each his own :)

: So, your criteria are: Mech that is not designed for a Spartan.
: Mech that is designed for a Spartan.

: Anything more specific?

Nope. In the context of Halo, I can't feasibly see the existence of a mech that's designed for a Spartan which adds nothing to the Spartan's Mjolnir capabilities.

A Spartan wouldn't/shouldn't use a mech if it doesn't grant some sort of capability or benefit beyond what the Mjolnir already provides - whether it's built for a Spartan or not. So,
1) a mech designed for a Spartan (grants more capabilities, not usable by non-Spartans) or
2) a mech not designed for a Spartan (which also grants more capabilities, but usable by 'normal' humans)

Discussions I've seen seem to be centered around how the Mantis and Cyclops fit in that context. Is the Mantis actually beneficial (feasible for a Spartan to actually use)? Does the Cyclops really have a believable place in the Halo fiction?