There's a few reasons to play the Sentinel as a cqc character, all of which a matter of min-maxing, so feel free to do it some other way if you like. Like capn233 said, AA is better up close than far away, but you also get an awesome CD bonus as a Sentinel, meaning it's good to get in close to spam your otherwise weaker spells. I personally find most biotics are only good relatively close up, since their projectiles are slow.
As for the god-build, the GPS is preferred for its versatility: high shot damage, stagger, ammo efficiency and relatively long range. Assault Rifles work, but they compete with the Locust and none of them can OSOK like the GPS can. Sniper Rifles don't mix well with biotics, again because of slow projectiles.
Taking Power Armor improves TA's synergy with long-range playstyles, but IMO its bonuses don't amount to as much AA's do. IMO, the ME2 Sentinel is just better suited to cqc builds than to long-range builds.
Before you get the good upgrades and max a power + passive, most classes are pretty weak. Sentinel is not an exception.
That's not really accurate. Different classes come into their own at different levels, which changes their difficulty curves. The caster classes start off much weaker than the combat classes.
Soldiers, Infiltrators and Vanguards can get by pretty well with just their class powers maxed out, and their additional weapon(s) help make up for the lack of abilities with extra firepower. At the very least they're getting an additional 33-50% ammo capacity from their extra guns. They can also run away or increase their toughness/damage whenever they're in a pinch.
The caster classes need their class powers maxed out as well as at least rank or two in all their debuff/CC powers in order to start becoming effective. This is mainly because their starting weapons are limited and they can't really afford to spam bullets from a safe distance nor any way to escape danger (eg: Charge):
- Engineers have it relatively good; as Combat Drone does so many things. They just need a couple points in Overload and Incinerate and they're good to go as far as 'controlling the battlefield' goes.
- Sentinels need TA maxed ASAP, then they need rank 1-2 in Warp, Overload and Throw. Otherwise they're only good for sitting around and getting shot at because they have durability but no firepower nor CD bonuses.
- Adepts need Singularity maxed and then they need basic points in Warp, Pull and Throw before they really start becoming practical. Weapons are their main debuffs, so ammo will be an issue until the Collector Ship. I'd actually argue that Adepts are the hardest class to start on Insanity with because Singularity is relatively weak and Adepts are meant to use a lot of powers at once, while not having the kind of survivability that Sentinels do.