So, I have been planning to run as a Medium Operative for a handful of reasons:
~I figure I will be fighting mostly against Light characters; as a Medium I will have more health and I will have heavier weapons to take them down faster in the close-quarters combat I'm expecting to be in a lot. I may not be as fast or as amazing with SMART, but I will certainly be able to get everywhere I need to be pretty quickly. I'm just not going to consider playing a Heavy so let's not even touch that.
~As an Operative, I understand it's not a badass assassin class, but more of a hacker class with certain stealth elements being higher priority than other classes. As for objectives, I figure there will always be Command Posts to hack, if nothing else, but I doubt that there will be nothing else (turrets can be hacked, disguises can be used, etc etc, everyone knows how great and cool Opertives are, moving on).
However, as I am re-reading the extremely detailed http://www.fragworld.org/frag/brink/the-fragworld-brink-compendium.html, and playing the demo to Enemy Territory: Quake Wars (another squad/class-based FPS made by Splash Damage), I have developed some serious second thoughts as to my selection of body type and class.
~Everyone knows Lights have the greatest mastery of SMART; that's why 99% of the players who plan to run Lights are doing so. I don't need to jump full stories so I'm down with losing some capabilities. But wall-hopping is a Light-exclusive move. I'm not sure I want to lose something as potentially useful (in both travel and combat) as that, but I also realize as a Light I would lose a good amount of health and access to assault rifles and shotguns, which I really want to be able to at least try out and see if they are as good as I hope they are.
~I'm not planning to run Soldier at all; I'm probably not going to live long enough to run out of ammo, and blowing stuff up isn't my style.
~Playing a Medic was wildly fun in the ET:QW demo just because I could run around and keep my teammates alive while we simply destroyed the enemies trying to reach and complete objectives. Plus, Medics will always be needed to revive and support players.
~I like the idea of Engineers buffing weapon damage; so long as I get to buff myself too (I didn't see anything for or against that in the Compendium), that is actually right in line with my combat ideals next to stealth (one of my earliest Abilities is probably going to be Running Silent, which is comparable to the Ninja Pro perk of Black Ops) and close-quarters combat (in Halo, my top weapons are consistently CQC-based; melee, AR, and shotgun are three of my top five kill weapons). But, the counter to being an Engineer is that any turret or mine I place could be compromised by the infestation of Operatives that will swarm Brink on launch, so I would be restricted to specializing in buffing weapon damage, repairing/constructing objectives, and disarming Soldiers' charges (one of the things Operatives can't completely hack), at least until the majority of players find their true niche. On the plus side, Engineers will likely always be in demand and always have great objectives; kinda like Medics in that regard. On the flip side, I felt pretty helpless trying to complete Engineering objectives with no teammates around and always got destroyed when I tried to build stuff in the ET:QW demo (see why I liked to play Medic, above).
I'm well aware that Operative is the most prevalent class here on the forum, trailed by Medic, and I've rarely seen Soldiers or Engineers floating through, so obviously people will push for changes to Soldier or Engineer, but please keep in mind my stated concerns and playstyle. I want a bodytype and class that is best fit for me, not what will let you keep your bodytype and class so you don't have to worry about not having a [insert what your team needs] on your team.
So. Convince me. Medium or Light? Medic, Engineer, or Operative?
Warning Part 2: Don't expect me to acknowledge your existence if you pull a "TL;DR" and then ask a question that was answered in the OP: you were just too lazy to read it for yourself.