There are plenty of tower defense games out in the wild that innovate the formula, change, and remix everything; Defense Grid 2 isn't one of them. Instead the game offers a polished and well-designed experience for new and experienced players with some genuine difficulty thrown into the mix. Hidden Path Entertainment knew they had something good on their hands when they released the first and their kickstarter allowed them to build up from the original game's framework into one of the smoothest tower defense experiences you'll ever see.
DG2's gameplay is simple and basic, essentially a paint-by-the-numbers tower defense game, which isn't going to appeal to you if you aren't a fan of the genre. However, if you DO like tower defense games you'll find yourself well-pampered and well-acquainted. Everything beyond the basic tower defense framework - IE build towers to murder aliens and protect your macguffins - is designed to improve the quality of life, with visible firing ranges, a timeflow control, and special abilities allowing you greater overall control over the game. In fact despite being deeper overall, TD series like Orcs Must Die and Sanctum don't actually come close to as smooth and polished of an experience, while games like Yet Another Zombie Defense manage to only just barely fill the blanks and end up too barebones to enjoy properly. The online play manages to put it a bit above the rest however. Versus mode plays out like a frantic tower defense version of Panel De Pon's while co-op is actually fairly balanced and manages to avoid trivializing the game. From my playthrough the netcode worked out fine, I was playing with someone halfway across the world with little to none noticable latency issues.
The sound design is average at it's best, with the addition of more voice actors beyond General Fletcher (which SEEMS to be the same voice-actor; Jim Ward,) though none manage to be as entertaining, while Fletcher's constant quips manage to be the ONLY reason to keep sound on. Unfortunately most of the other voice actors natter ceaselessly and drive you to eventually - despite the genuinely funny Fletcher - shut off the sound entirely.
The visuals are unfortunately inconsistent, bland, and seem to be merely the bare minimum save for the occasional well-sculpted environment. Nothing truly stood out to me visually, and the game managed to chew through resources at an absurd pace for what was actually on screen.
While nothing like the bizarre opus Immortal Defense, DG2's story ends up feeling odd and disjointed. Everything I've seen points toward them having to cut a good deal of story to fit their final budget - the kickstarter made $271,726 out of $250,000 regardless of the outside funding they got - and left a poorly placed mess in it's stead. What's worse is that the game relies a bit on the framing of the first game & the DLC that game has to bridge the gap between the two and practically throws you into the game with just a tutorial and no explanation otherwise.
While generally proficient, Defense Grid 2 isn't as good as the first and doesn't have the same appeal. It's still a good game, but it isn't a great game and DLC probably won't make it one. Even so, it's a strong and well-designed game that focuses on what makes tower defense games fun and it has a leg up on most steam tower defense games. The price tag may give you pause but if you like the genre at all you won't be disappointed.