Update on current state of the game for folks that might've been on the fence:
They've finally introduced 12v12 combat. IMHO, it's fun as hell, but YMMV. Some folks feel its too crazy on the smaller maps, but personally I'll take nonstop action over trudging 3 minutes before seeing your first mech. Some folks on lower end machines reported lag with 12, but after recent patches I've heard less. Pre-mades can drop as 2-4, or 12. 12v12 has not eliminated the problem of pubbies playing premades, but it has diluted it somewhat. The ELO system seems to be fairly effective, most of the time. However, the nature of a game with no respawning means that the first team to lose mechs is generally going to snowball into a bad defeat, and 12v12 makes it more noticeable. But hey, quicker matches just mean more matches, and it makes the close matches all the sweeter.
Launch/main UI is largely unchanged. Still the same clunky Ready/Launch interface, the same crappy chat interface, the easy to miss 'invite' notification. If you hated it then you'll hate it now. The mech lab itself is improved but still clunky. In-game UI however I think is pretty decent with some nice improvements.
Weapon balance is probably at its best point. PPC stacking now leads to huge heat penalties. LRM's and SRM's seem to be at a sweet spot where each are deadly under certain conditions, but smart piloting can reduce their effectiveness. Even MG's are a fair use of tonnage. The laser family are probably weakest at the moment, and PGI has acknowledged it.
ECM has been rightfully nerfed. Beagle and TAG are now effective counters to the nastier features of ECM. It's still a worthwhile edge, but with the various counters it no longer overwhelms matches.
Mech balance is pretty decent, while some mechs are favored more than others you'll still see virtually all of them over a series of games. If there are any complaints about mech balance, it is a usually question of mech classes - assaults are still predominant (but larger maps are shifting this dynamic towards heavies), lights still seem to have a little bit of a magical lag shield around them (but much less so than in prior versions).
Slope changes were introduced, to generally negative reaction. You slow down and even stop on the steeper map slopes, which makes sense but plays in a frustrating way. Expect lots of sliding around trying to find the magical angle to get up a steep slope that you previously had no issue with. (on a personal note, this change only entrenched my dedication to jump-capable mechs)
New maps are a bit disappointing. The latest, the Terra Therma volcano map, is beautiful but frankly the worst multiplayer map I can recall in a game in a long time. It is obnoxiously large, so large that 12 mechs can easily pass the other 12 mechs on their way to each others bases, and never see each other. Despite the size, the map feels heavily constricted, forcing mechs through narrow chokepoints, so when you do encounter the other team you'll spend a lot of it firing into your teammates' back or vice versa. The other recent maps, Tourmaline and Arctic Bigass Map (name escapes me), are much better off for 12v12 action, but only Tourmaline really feels much fun to play. It has the right blend of openness plus cover, IMHO. Likewise for Canyon, but Canyon suffers from the new slope changes. If any map makes you grumble why you didn't go for JJ's in your mech, it's that one.
Game modes are still the same with nothing new planned any time soon. They've greatly increased cap times, which has greatly reduced the "back-capping" phenomenon that ruins the game, but at the same time makes capping a little bit tedious leading to 'Stand still at a base for one minute, or go hunt mechs?' decisions. Overall I like the change, but would prefer to see nodes fought over at the center of maps instead of in the rear.
Economy still needs work. Consumables seem to be a bust - I never see consumables used, at least in public play. With 12v12 matches you earn less c-bills now than in 8v8, however, so the grind has gotten longer. Lots of complaints there, it's possible they may tune that up. However, there is still little evidence of P2W taking over. My biggest complaint on that front is the extremely expensive modules that require GXP to unlock (free players must play a ridiculous amount to build the 5000-15000 GXP needed for just one) and mucho c-bills to purchase (~6million). Modules give players a decent edge - particularly the newest one, Seismic, which was practically a wall-hack in usefulness until it got nerfed. Even in its nerfed state, it's an indispensable module to have. Hero mechs ($$) remain pretty balanced with their 'free' counterparts.
Speaking of Seismic, and the map Terra Therma: these were such horrible implementations that I really worry about how the dev team vets new ideas. Basic play-testing should have revealed Terra Therma's problems with size and constriction. Basic play-testing should have revealed Seismic to be ridiculously more useful and powerful than any other module. While PGI has demonstrated a decent track record at balancing problems, the fix schedule has proven to be pretty glacial. ECM took forever to balance, Seismic went months before tuning. Terra Therma is so bad that for the first time, I wished for the option to exclude maps to launch into.
Stability - I play with 3 others regularly. One buddy and I never crash. One buddy crashes out once every three matches or so. The last guy on rare occasions disconnects at match start (he has a dodgy connection) and we haven't seen that issue in a few patches.
Overall though the game is still a lot of fun. I'm often surprised at the disparity between this thread and Planetside 2, which I think is just as flawed (and fun) as this one.
Anyway, hope this summary of changes was helpful. Happy hunting.