So I figured that it was time and finally downloaded this. Any advice for a new player?
Try out classes until you find one you like and stick to it until you feel comfortable with how the game works. As stated, stay away from 24/7 servers, the players tend to be dickish on those. It's not worth it to spend money buying guns, since those just drop/are gained with achievements quite regularly. You'll probably only need to use the mute menu if some kid starts screaming in his microphone. But that can lead to amusing situations (had a kid start discussing with another regular at a server I frequent, eventually the kid's mom stepped in and started arguing with the now-clearly-trolling regulars).
And yes everyone is a spy. Even you. Spycheck regularly.
Actually, spending money ONCE is actually a pretty good idea because you get a much larger inventory and tradable drops. But I'd spend it on a key or something (that you can trade for lots of weapons or HAAATTTSSS) instead of on weapons.
Just a note here if you are going to purchase items, you should usually only purchase a single item from the in-game store, in order to unlock the larger inventory and trade functionality. Pretty much anything else you will be able to purchase cheaper from using the community market. If you have any games that give you trading cards, you can usually sell them for enough to make that first purchase without having to actually put money on your account.
Played around a bit on the new beta maps. Cactus is great for rocket/bomb jumping soldiers and demos, particularly with the new parachute. Once in the air, you usually have fairly clear long sight lines in most points of the map. I suspect invisible walls will be erected over many of the points you can currently jump/float over though. It does seem to be a bit too easy to defend at certain locations though, although that may be partially down to few people knowing any of the alternate routes where they are available.
Asteroid is a fun variant on ctf, but needs better signposting/props to identify where you are, may be improved when the temporary wall textures are replaced. It also needs a directional indicator, like in ctf, towards the enemy core carrier. The doors that open as the bots at one of the points are destroyed need to be more clearly indicated. It is too easy for the spawn exits to be camped. The lower tunnel could do with at least a few pieces of cover along it's length, too much of a sniper gallery at present.