This is my guide to playing as the Sentinel class in Mass Effect 2.
There are three abilities that you need to know about before diving into the Sentinel class:
1) Warp - This biotic power is available only to Sentinels and Adepts, and it is a force to be reckoned with. It deals double damage to barriers and armor, which makes it a key piece of any run with the Sentinel on higher difficulties. It also halts health regeneration. Any Sentinel will be using this ability very often.
2) Overload - This tech power is available only to Sentinels and Engineers. It is a highly important skill, as it can shut down enemy shields, which makes them very vulnerable to attack. As many enemies are shielded on higher difficulties, any Sentinel must be familiar with Overload.
3) Tech Armor - This is the Sentinel's bread and butter. No class but the Sentinel can use Tech Armor. While the first two abilities listed here are used for when Shepard goes on the offensive, Tech Armor shields Shepard to attack. Shepard with Tech Armor active can withstand more than any other class. Also, when Tech Armor is destroyed, it sends out a pulse of energy which damages and sometimes knocks down enemies.
Also, once Tech Armor has been improved to Rank 3, when it is activated it resets the cooldown for Shepard's squadmates. This makes the Sentinel's squad much more efficient than that of any other class. Tech Armor is wildly important to the success of a Sentinel.
In addition to the three most important abilities to a Sentinel, they can learn Throw as well. It isn't an especially effective ability on higher difficulties, as enemies are shielded, but against a group of Husks it still has its uses. Crowd control is important in those situations, and Throw is a good skill to have for that.
Sentinels can also learn Cryo Blast, but compared to the powers listed above, it is unnecessary. Warp and Overload take care of all the offensive abilities that Sentinels need.
There is also a passive power called Defender that essentially works like any other passive power in Mass Effect 2. It is mainly used to increase the character's health and to improve the recharge time for the Sentinel's powers.
Weapons are the only thing that could be considered a weakness for the Sentinel class. With only handguns and machine pistols/submachine guns available to Sentinels at the start, they are lacking in firepower somewhat. That is why when I am playing as a Sentinel, I complete Kasumi's loyalty mission right away; there is an outstanding submachine gun available there. It makes up for any perceived weaknesses in their arsenal.
Then, when the crew goes to the Collector Ship and Shepard is allowed to choose a new gun, I recommend taking the Assault Rifle. It adds a new dimension to the Sentinel's arsenal, being a gun that can be used up close and at range. It turns the Sentinel into a more diverse weapon.
Let's review: the Sentinel is an exemplary class because it protects Shepard more than any other class, it boosts the effectiveness of his or her team, and packs quite a punch when it needs to. There is no class I'd rather be on a suicide mission.