All About D3 Defenders
Pros
As a defender, your variety of gameplay is astounding, you can solo very well, but you're best in teams. You can heal, you can debuff, you can control, you make a good support character, you make a good leader. You have a power for every situation.
Note that you have one of the only powers in the game that gives Psionics resistance to you and the ONLY one that gives Psionics resistance to your whole team.
You will go through the game in relative safety due to your secondary effect of reducing target accuracy. There are times you can take a break in the middle of a battle and come back with enemies still whiffing trying to hit you. Darkness powers also work very well in character design because there are themes and story plots that just work with a dark bent.
Cons
You're weak. Face it.
Your defenses aren't strong though this is mitigated slightly by having a heal and a pet who can heal you too. But your attacks are weak too. Granted you can keep enemies from hitting you at all, but it still can take a long time to beat enemies down yourself. With at least a few extra teammates to do some damage, you can kick some good butt though.
The most serious drawback to a D3 that takes getting used to is that many of your powers require a "victim". For example, you have a pretty good group heal and the best Rez power in the game by far, but you have to have a targetted enemy to use them.
Summary
You'll often be the puller (since you have Debuffing attacks and the longest Snipe attack in the game), you'll be the one who opens battles (with Howling Twilight, Tar Patch, Darkest Night), you'll be the tide-turner (when you fire your mid-battle rez or drop a Tar Patch to cover your team's escape), you'll be the one who cuts the TF time in half with your Sneak and TP combo, and you'll be one of the fastest characters around which will let you TP your team mates to missions who chose other less quick power combinations. Your contribution to teams will be immediate and obvious. When you take a Bio break, your team will notice the difference.