It really lacks depth in the early game - however when it comes to finally unlocking shit, it gives you quite a few different options that whilst seemingly subtle can have a profound effect on how the game is played. At Bronze level the game seems pretty badly balanced, because quite a few of the classes don't fare well without alternative unlocks. At Silver level you'll see a lot more variety - I recently started playing at Gold level and when playing matches, usually every class (or almost) is represented on my team - I'd call that a success in class balance.
The level design isn't great, but it is at least functional and has variety. Capture the Flag in Ascend is pretty naff imho (and Arena is pure sucktastic), but Capture and Hold works really well. Shame you have to play for 10-15 hours to unlock it, but then it's also best played with access to a variety of classes.
None of the classes are useless:
The Pathfinder is by far the fastest class and can be kitted out for either chasing (with the ability to launch at 140kmph from a standing start) or capping (thanks to high energy and strong benefits from splash acceleration).
The Sentinel is obviously a sniper, and in CTF games should occupy high ground where they can use their instant-hit weapons to stop cappers or chasers in their tracks. In Capture and Hold they can use claymores to defence and their range and accuracy to harass deployable and Technicians.
The Infiltrator is very useful and would always (imho) take the grenade launcher - in CTF they should destroy the generator and other base defences, but they can also harass heavy defenders something chronic. In Capture and Hold they should be identifying weakly defended points - usually those guarded by Technicians, deployables or singular heavies. They are the only class that can saunter past Turrets and Light Turrets completely unnoticed, and they are also rather good for hunting down and eliminating Sentinels.
The Soldier is a jack-of-all trades - I have little experience with this class, and I have to admit I don't see it used much.
The Technician is what I generally play, because it has some great tools. I play with the SMG as it is great at whittling down and finishing enemies; usually after they have gotten too close to a turret, or one of your three grenades. I mainly play Technician, as I can play both offensively and defensively, as well as still keep racking up a score and doing something useful when there's noone to kill or no objective to handle. Motion Sensors near flag stands are great for sucking energy out of cappers; so if they are deflected they won't be able to get away, and they can also deny Raiders the ability to use their shield, or Infiltrators their stealth (although both are usually likely to just destroy the detector). My main strategy is to locate isolated heavies, place a turret, then lure them into it and catch them in the crossfire. If they are aiming at me, I can keep moving near the turret and they'll take a lot of damage, if they go for the turret, I can chuck grenades at them to deal 1000 damage a piece. I spend a lot of time moving between crowded places and picking off stragglers, and defending points - the latter is important because without the inventory stations, ammo for the SMG runs out quickly and it is not a great weapon by itself unless at very short range.
Now that it has the Plasma Gun, the Raider is ridiculously versatile, but the main perks are the Shield (meaning you stop taking damage against health and take it versus energy instead), which gives them obscene staying power - especially when combining it with Potential Energy, which basically means you not only take damage to energy instead of health, you gain energy from taking area of effect damage.
Juggernauts have the Fusion Mortar which is perfect for bombing the enemy base at long range and excellent with area denial - and requires no fiddly perks or anything to do so, and also carries a nasty Spinfusor by default so it isn't vulnerable either. This mans you'll see a lot of Juggernauts, and with the scoring bias geared towards kills more than anything else, they will often take high scoring places in the leaderboards (even if they don't actually perform a massively useful role all the time...)
Doombringers are one of the weaker classes, but the Minigun is reasonable for spattering incoming cappers, the lock-on rocket launcher is largely useless (doubly so given how easy it is to shoot down the rocket), although it can encourage players to stay low. Using the chaingun, force fields and mines, it makes a good choice for 'Heavy-On-Flag', especially with the SuperHeavy perk.
The Brute deals high damage, but I find it is generally a weaker option at range than the Juggernaut. Indoors it's a clear winner - Fractal Grenades are ridiculous area denial tools that are perfect for clearing entire rooms and you can carry three of them. The auto-shotgun is also pretty nasty versus any enemy caught at short range.
Social interaction is indeed where this game is fucked. In an F2P game, it's difficult to actually get your buddies together into a proper game, and there's nothing like an internal messaging system, so even though you can add people to your friends list, you cannot ever communicate with them ouside of a game.