High Reflex Doth Not A Tank Make
Having a high reflex doesn't count for anything extraordinary. It's one of four defences beside armor, fortitude and will, and one quarter of attacks missing you more often doesn't make you a tank.
The magic of defenders, however, is not a matter of being unable to be hit. Defenders in D&D 4e take their denial abilities worlds beyond those of MC Hammer’s, and interpose themselves between their allies and anyone who wants to hit them.
A foreword: D&D 4e actually has roles to guide you here.
Contrast to previous editions where what a class should be doing was up to the players to work out, D&D 4e introduced the concept of roles: each class has one role, which marks something they naturally do very well due to the powers and class features made available to them.
- Strikers are very good at dealing high damage to a low number of targets.
- Controllers debuff enemies, or control where they stand by turning all the other places into clouds of knives or fields of fire, when they’re not just dropping explosive AOEs.
- Leaders buff their allies, heal them, and let them do extra stuff they couldn't otherwise.
- Defenders are good at keeping enemies away from their allies, punishing enemies who hurt them anyway, and generally keeping their allies alive.
These roles usually suck at the other stuff (short of extreme optimisation), simply because they aren't given much to let them do it. An ordinary controller or striker won't be able to buff or heal their allies much, and a defender won't be hitting as hard as the striker.
Rogues are strikers, not defenders. They're not equipped with the stuff that makes someone good at defending.
What makes defenders naturally better at tanking than other classes?
Let's take the PHB1's classic defender - the Paladin - and see what makes them so suited for the role of defence compared to a rogue.
There’s a few statistical things - Paladins have better defences and armor, more health, and more healing surges, but these aren’t the key. They also get powers to heal allies and make everything harder for enemies to hit, but this is only a contributor to the ultimate power of what makes a defender a defender. (By contrast, the rogues just hit things harder, and get to move around the battlefield safely in order to hit other things and stay alive.)
Defenders force enemies to make bad decisions.
This is the key thing defenders do better than anyone else. The key to defending is not being hard for the enemy to hit. Rather, it’s the interaction between the defender, their enemies and their allies which creates scenarios in which attacking the defender is the best option - but not a good option.
Enter the Paladin’s Divine Challenge.
The Paladin has an at-will power called Divine Challenge, and they can keep it applied to any one enemy so long as they focus on that enemy. For as long as that enemy's Challenged, any time it attacks anyone but the paladin, it has a penalty to its attack and automatically takes a reliable chunk of damage.
Thus, the enemy could either pay attention to the Paladin whilst being lit on fire by the Wizard, or they could try to take down the Wizard, but be bad at it and get hurt in the attempt, in addition to the fact the Paladin's still beating on them.
Or the Fighter and its Combat Challenge.
The Fighter, the other PHB1 Defender, fulfils its role through mark punishment: an enemy attacking not-them imposes attack penalties and invites retaliation from the fighter.
Fighters are specialists in close combat, and through target- and movement- denial, they’re the kings of lock-down. At level 1, a fighter with positional advantage can lock down a hallway by physically interposing himself between his enemies and allies. Anyone trying to shoulder past him gets stopped in their tracks.
BESW and trogdor, two of our regulars, together developed a build which combined the fighter's lock-down with its ability to push enemies around in order to lock creatures down way over there. It was frightening, and enemies targeted by this could do little more than cry in a corner until the party had dealt with everyone else.
Rogues don't really get to do this.
Rogues don’t have punishment for attacking not-them. They can pick up a couple of specific abilities which grant it, but it’s not baked into the class enough for them to be effective at it consistently for long periods of time.
If a fighter stands next to you, you’ll either attack him or get beaten to a pulp by him. If a rogue stands next to you, you’ll get shanked whether you attack him or not, and he can’t do much about you attacking his friends instead of him.
So, if you're a rogue, put pointy things in enemies. Get yourself a defender for the tanky stuff.
Class roles should be used to their strengths, not fought against to try to fulfil a different role the class won't do very well anyway. Rogues have their strengths - they’re going to shank or poke arrows in people better than anyone else might - and as a rogue, you should exploit this strength as much as possible.
If your party needs a tank, they should look into getting themselves a defender.
Bear in mind, Defenders can be stealthy too.
Since Stealth is a skill available to everyone, there’s nothing forcing you to choose between stealth and defending: you can do both! There are builds available for defenders who want to be sneaky, too.
- The Pocket Protector, a sneaky pixie Knight build with a whopping +42 to stealth by the time it finally reaches level 30. That's the Fighter (Knight) from D&D 4e Essentials.
- The Trolladin, aka Troll-lock: Play a Warlocked multiclassed into Paladin, or vice versa. Pick the Fey Pact to gain Eyebite. Use Divine Challenge on an enemy, so that they'll be punished for hitting anyone except you. Then use Eyebite on them to make yourself invisible and your ability to teleport to put yourself out of reach. The result: they can't hit anyone else, and they can't hit you, either. (This was discovered and handily employed by trogdor, who I mentioned earlier)
(And there's probably many more than that.)