Most of the information is not detailed enough but I'm just trying not too spoil too much
2) Are the game's missions varied?
They're okay. Most missions are the usual "shoot everything that moves for a while". Some missions like The Portland Chainsaw Masquerade, Taking The Peace, Steering The Vote, Karmageddon and Bringing the House Down are fun and unique.
3) Are the side-missions good? Can you guys list some of them?
They're alright. Some of them are fun and varied.
Well Snacked Pizza
RC Bandit races
Car & Bike Salesman
See The Sight Before Your Flight
Car-Azy Car Giveaway
Bumps and Grinds
Then there's the missions from every other GTA - Paramedic, races, etc.
4) Can someone please list the full features of the game? (Or at least the main ones)
No access to helicopters.
Liberty City from GTA III but with a few differences. Mainly everything is under construction and Fort Staunton is not a construction site.
You play as Toni Cipriani.
You are a member of the Leones so they won't kill you whenever you enter Saint Marks. They all carry pistols anyway - just like every other gang in the game (minus the Colombians and Diablos).
Exclusive weapons (Chisel and Hockey Stick).
For a part of the game, a hit is called on you so from time to time, there will be a Hitman you can kill to get weapon and armor.
5) I also am thinking of 100% the game... any tips?
Do everything possible in the beginning. Gangs are pretty much harmless but they can still be a pain in the ass from time to time. Paramedic is easier than in III/VC. Some side missions have to be done 3 times - once on each island. Hidden Packages are rewarding (First few are quite bad but the rest is nice, in my opinion).
6) Is the game worth getting in general?
In my opinion, yes. The game may not have a lot of features but it's still really fun and unique. Toni is a total badass, Sal is the best main character in the game and Donald Love is a wierd cannibal who is nothing like the Donald Love we know from GTA III. Vehicles are fun to drive. Hellenbach GT for example is a very fast car and good enough for crazy stunting. Also this game has helicopters and you can't get them but with a few clever tricks, you can get one to fly any time you want. Side missions were the most fun and unique side missions I played. No other GTA allows you to drive a garbage truck and actually pick up trash and get paid for it. Now if Toni had a garbage collector's outfit like Niko does in IV, that could be great for roleplaying. Avenging Angels are good missions that last however long you want and they are actually varied (unlike Vigilante) so they're good if you like to grind for some cash and kills. If you want some legitimate source of income, you can sell some cars (PSP version can be only done once... and it's limited) or bikes (infinite on all versions) or you could sell some Noodles or Pizzas. It's your call. As I said, gangs are pretty much harmless. All of them carry pistols except the Colombians who actually care about defense and carry Micro SMG's. The gangs are like in GTA III. Everyone will hate you in the end other than the gang you belong to and the Hoods. The story is interesting enough. You're basically trying to get Sal and Leones into running the city but it f*cks up and then you blow up a tank, mess with Yakuza's, blow up a bunch of buildings, save Sal from getting killed (again) and then you finally take over in a similar way to GTA III (aka: blow up a helicopter circling around you with a risk of getting killed quickly). Missions are very fun but as I said, most of them are "kill x" or "drive to x". As for weapons, there is a big variety but I always end up using SMG or M4 anyway. Rampages are easy enough and are actually rewarding (you get an M60 to safehouse after completing all of them). Anyway, a lot of this game is fun. It doesn't have a lot to show off and can sometimes get boring very quickly but it certainly is underrated. This was my first GTA and after playing it, that was the only game I actually played.
So, yes. It is worth it and I recommend getting it.