You don't want to explore because of an achievement? Are a few gamer points
that important to you?
If most of the places look the same thats because you probably have your eyes shut.
If you could tell where you were and knew exactly where to go without looking from the map on day 1 with GTA 3/VC and SA I tip my hat to you but I honestly think thats a pack of lies.
All of the cities are confusing and large at first but with some playtime and exploration (which you don't want to do because you might get 30 gamerpoints!) they become less overwhelming and much easier to navigate.
For example you could easily have done some taxi missions for Roman or maybe some police missions from the police car computer (the police stuff gets you Gamerscore too you know

)
As for finishing the main storyline and then loads of side missions before jumps/packages for 100% I can agree to a point as there are fewer side missions but what is there makes heaps more sense.
Brucie wants to go racing, Little Jacob wants you to deliver drugs and Roman wouldn't mind you helping with his business. As I mentioned police work is available and aids you knowledge of the city and seems to be available in a much more realistic fashion.
I miss being fire proof and having extra health but I don't for a minute miss Fire missions or Ambulance missions - the way those played out made little sense. A heavily muscled gangster driving an ambulance of old ladies whos's apparent injury is nothing more than a cut knee to hospital for 12 'levels' was monotonous.
As for multiplayer have you actually played it at all!? The game will allow you to play in areas as small as Bohan and as large as the entire game map with no restrictions. Xbox Live certainly can support these large play areas and 16 players is quite frankly plenty for some competitive play thats still contrallable.
A point made by my father (who just happens to love the GTA games and has finished them all bar IV) also reckons from reading your gigantic, hard to read non stop typing whinge that your not really seeing the point of how the game should be played.
Side missions should in all honesty be started early and played often until you have the mechanics of the game sorted and can find your way around. By all means do some missions too.
Getting some of the side mission bonuses early on can be an invaluable help (Jacobs reduced weapons for example) as money can be tight to start with and you don't know where you are going.
Also with added exploration you can find plenty of weapons and armour lying all over the place and can therefore tool yourself up for free and not incur too many expenses.
Finally I believe (although probably not a popular belief) that the 30 hour achievement should be one of the last ones you do as by the time you have finished the game you will have enjoyed and experienced all the detail the developers worked on for YEARS.
Oh yeah random smiley so life feels good