Sentinel is the only true sub class in the game but with at the moment there are many issues to look at about this class.
1) The way skill points are gained for this class are unfair to other classes. The amount of skill points gained are based on the rank of the main class and not the rank of the player's sentinel. A rank 1 gladiator can continue to raise the skill points of their rank 49 sentinel off level 1 monsters which was achieved through another class. The calculations for skill point gains should be determined by the rank of the sentinel class against the rank of the monsters.
2) The number of classes that can receive skill points for sentinel are too many. The most common rank 50 class right now is undoubtedly sentinel. With the incorrect skill gain and three different classes able to rank it with out any extra work is quite unfair. Future one-handed weapon DoW and DoM classes will likely be able to equip a shield as well unless drastic discrimination is added to the current shields to change who can wear what with future class releases. With all the problems stated above many people will have a meaningless rank 100 sentinel class
3) The rank of the sentinel class has absolutely no effect on the quality of any shield related skills or actions. Ranking up sentinel gives you seven skills and the ability to guard/block and that is all. These skills mostly benefit gladiator which already has a few shield skills and traits of its own. How long a player blocks is determined by wield rate. How often and how much a player blocks is by shield type and just having a shield on is additional defense and magic defense. The current sentinel skills could be given to gladiator through ranking up gladiator and be erased completely and it would be no different.
4) I cannot see the progress of my sentinel skill rank at all in game. There is no <exp/skill> command to display such information and the rank and level bars never display for sentinel because it is not a main class. The only way to check such information is through the lodestone character pages.
Overall, Sentinel just really feels like an incomplete class that could easily be assimilated into the gladiator move set and be erased without much complaint.