As a player who began their HAWKEN journey when the game came to STEAM and has tried and failed to get many friends into the experience let me share my experiences:
Complaints by friends/acquaintances in order of frequency:
1. The free to play model is out of line with other free to play experiences
             Simply put, everything is far too expensive and the customization system is unappealing to many players (somewhat to myself as well, however I stick with it).  The most excessively expensive items seem to be, well, items...  Players don't want to grind for hours and hours to be able to afford an item that they may not even end up using in the long run.  Items like MG-Turret are SOOOO expensive that some players save up to get it with a certain expectation, and then when it ends up being utterly useless they feel ripped off and unsatisfied.  Internals aren't too out of whack, but items are, and so are alternate weapons - being as expensive as just getting another mech.  Many friends have told me "Why would I buy an alternate weapon when I can just get another new mech?".
             The customization system is also really out of sync with a feeling of "value per dollar".  Unlocking a set of customizations for a single mech can be excruciatingly expensive, ESPECIALLY since you can't share seemingly class universal parts between mechs of the same class...  Here's an example of price point comparison.
Here we have the Grenadier:
 
Here we have the Grenadier with a kind of "Sand Shark" customization scheme.  Brass Shark Upper, Muklashy Arms, Lower, and Middle, Pixel Tan Paint, Ignite Thrusters, and RD-L4NC3T Drone:  2404 MC or $16.69 USD (based on a model of 144MC/$1 per the $25 - 3600MC price point)
 
Look a 
little different, enough to feel customized.  
Now, what can we get for customizations for $16.69 in other f2p games?
Let's take League of Legends and DotA 2, two of the most popular f2p games:
In league for $25 you get 3400 RP, enough to purchase 3 full price skins and 1 cheap skin/1 on sale skin.

As you can see, that's 4 different looks for the same character for the price of 1 different look for a mech in Hawken.
DotA 2 has an even more cost efficient model, using armor "sets" similar to chassis parts in Hawken, but with more variation, colours, and overall more value.  Again, like League, you can get 4 sets for rougly $16USD.
 
Also, with both League of Legends and DotA 2, the only thing you'd really NEED to spend real money on is customization, and not functional items and "pay to win" material.  League of Legends actually makes it so you CANNOT buy runes with real money, dissuading any idea of a "pay to win" environment and giving players something that they can feel good about grinding towards.  DotA 2 has no runes, no masteries, and all the heroes and permanently free to play, meaning the only thing you ever need to buy are customizations.
Also worth noting is that League of Legends has a built in refund system and DotA 2 has a marketplace system, meaning if you buy something you REALLY don't end up liking you can either fully refund it, or trade it with someone else.  Both games also have a gifting system, allowing players to help each other out with new content.  This is something Hawken REALLY needs, especially for people like myself who are sitting on a garage with 10 Technicians in it (one from every $25 MC card...).
I've had a couple friends who have quit Hawken solely based on the principle of "pay to win", seeing that I have a fully outfitted mech at rank 1, with MK-III items and Internals and they have a Fred with nothing on it.  The sad part is that the "pay to win" system is currently needed to keep ANYONE playing the game with a new account, since new players are frequently matched with high level players with outfitted mechs and a LOT more skill.  Being stuck in a server with an empty Fred against 3 Berserkers with Air Comp/Deflectors/MK-III Det, a Brawler with Armor Fusor/MK-III Repair Charge, and a Raider with Air Comp/Deflectors, HE-MK-III, and Radar Scrambler feels like you're not even playing the same game (thank you based matchmaking).
This brings me to point number 2:
2. Matchmaking is broken:
           Many friends have quit out of sheer frustration at losing 15 Siege games in a row due to high MMR players with fully outfitted mechs taking a steaming fuzzy-bunny all over them.  I've experienced much of the same things, but luckily I'm a bit more patient than some and just really love the gameplay.  When you play for 6 hours and can't get your win of the day bonus it really does feel hopeless to say the least.  The worst thing is when high MMR players "toy" with "noobs" and allow them to hold the AA simply so they can pick them off over and over, never letting the game progress, but farming XP for as long as they can.
3. Lack of developer interaction:
           One thing that (hate to bring them back into, but I digress) LoL and DotA have that makes the community stick with them is developer interaction.  The developers are always on the forums, asking for suggestions, advice, and commenting on player woes and concerns.  The community gets a feeling that they're actually cared about and valued and that they can see the game growing and progressing and feel like they're a part of the development process (whether they really are or not).  This is WAY more important for smaller games like Hawken that are still developing and growing and need to develop a strong and dedicated community that feels loyalty to the developers and the game itself.  Without the feeling that the developers "care" about the community it is more difficult for players to stay committed to a game, especially one with as many glaring imbalance issues as Hawken (though larger more developed games are CERTAINLY not beyond imbalance and glaring issues).
Point 4 time:
4. Imbalance issues create pissed off players.
            New players having to deal with things like the Berserker hovering around their head and towing the fuzzy bunny out of them, the Incinerator's never-ending torrent of fire and bullets, and getting alpha goomba stomped by Raiders can dissuade players from even wanting to learn the game.  This ties into the matchmaking point, but the fact that even higher MMR players complain about many of these mechanics mean the lower tier players have no hope that anything will improve for them.  Getting outplayed is always going to happen, but feeling like there's no hope other than playing the same broken strategies can really dissuade new players from committing to an experience for any length of time.
5. Optimization
           This is gonna be my last point for now, and is one of the 
least important, but still complained about, is the way the games engine is optimized.  Games like League, DotA, Smite, SWTOR, etc... are able to scale down their graphics and effects and still retain the gameplay, allowing players with lower budget machines or just old machines to be able to play the game while still providing top tier visual aesthetics for those players with powerful rigs.  Hawken is simply poorly optimized.  Many friends have had to submit to playing at 20fps on low settings and never really enjoy the experience as a result.  Other games have a wide appeal based on the fact that they can be played on anything with at least decent framerate.  I have a 7 year old laptop that can run league at 50fps on lowish settings and a newer machine that can run it at 120fps on maximum settings, but I can still enjoy playing it on both machines, admittedly with a different aesthetic satisfaction.  My 4 year old desktop to the right of me cannot run Hawken at ALL, but can run DotA 2, League, Dawn of War II, and many other multiplayer games that I enjoy playing with guests.  It's hard to get friends into a game that can't be played as universally as other, more easily accessible games.  I understand the need for technological and graphical progression, but when it heavily limits the player base of a growing f2p game the trade off isn't worth it.
These are just some of the complaints/issues that other new players/myself have had.  I welcome comments and critique of my points, remember I'm a fairly new player, but I have spent a decent amount of money and time on Hawken so far and am fairly committed to it myself, but find it hard to get friends into the experience for some of the reasons above.
Thank you all for your time.