For Traps you definitely want to take Caltrops, Acid Mortar, Force Field Generator, Poison Gas Trap, Seeker Drones and Triage Beacon. I'd wait until the late 20s or 30s to take Triage Beacon but the others you want to take as soon as they become available.
Web Grenade is a nice power to have for AV fights in TFs but it isn't essential.
Trip Mine is a personal preference thing, you can reasonably reliably run into the middle of a group and drop one while your FFG and PGT keep you safe. I personally don't like doing this but my Trapper is Traps/AR so I get a lot more AoE damage from my secondary. With Dual Pistols however Trip Mine does offer superior DPA compared to your secondary AoEs so you should consider it.
Time Bomb is a gimmick power, take it if you really want but it's the first thing I'd drop in a tight build.
For Dual Pistols I'm less certain, here's my thoughts on the matter:
Pistols: Required so you might as well plan on using it
Dual Wield: Necessary if you want a solid attack chain but if you're planning to rely on toe bombing you can skip this, it will make you slower against bosses/AVs though.
Empty Clips: This is a pretty poor AoE, I'd say skip it if you're planning to go toe bombing else take it but YMMV
Swap Ammo: Take it, and learn the different debuffs
Bullet Rain: Not a great AoE but better than Empty Clips so I'd take it
Suppressive Fire: A single Target Mez, personally I don't find that Traps needs a blast set mez but it's a decent power
Executioner's Shot: See Dual Wield
Piercing Rounds: Definitely take it and set up some binds or macros to allow you to switch to lethal ammo, fire PR and switch back to your old ammo easily
Hail of Bullets: This does the same damage as Trip Mine so in some ways it's tempting to say skip it and rely on toe bombing but I'd probably take it since it has a larger radius than trip mines and isn't interruptable.
Nothing much to say on pool powers. Leadership is always nice to take and fighting will make it much easier to hit the softcap if you go that route. Hasten is definitely useful for Traps but is not required.
For Ancillary pools I took Mu Mastery since I just wanted the shield but YMMV there. If you want more damage I recommend Dark Mastery for a Trapper, you jump into a group lay a PGT and then use Soul Drain to buff your damage, not to shabby. Mace Mastery has a defense shield which makes it easier to hit the softcap without Fighting although if you take fighting I'd advise taking a resistance shield.
For IOs you want to build for recharge and defense. Traps needs all the recharge it can get and with the Defense from FFG (and fighting or mace mastery if you took them) it's not to hard to softcap it as well.
Time Bomb is decent if, and only if, you have some way to be invisible. That might be superspeed plus a stealth I/O (and Grant Invisibility for your FFG), or it might be something else.
The key is, a time bomb placed in a spawn can open the fight -- and you can be around a corner. Across a minefield. 
Good for soloing stuff that would otherwise be out of your reach.
I'm doing a traps/dp at the moment, and my build is focused in part on picking up things like resistance debuffs and defense debuffs, then picking up utility stuff like leadership pool, recall friend, etcetera.
About the slotting, I haven't really got any money, so I'm mostly looking for a build with SO's at the moment. Thanks for the suggestions so far!
SO build here for ya. Will give you some ideas hopefully.
Hero Plan by Mids' Hero Designer 1.91
http://www.cohplanner.com/
Click this DataLink to open the build!
Level 50 Magic Defender
Primary Power Set: Traps
Secondary Power Set: Dual Pistols
Power Pool: Speed
Power Pool: Leadership
Power Pool: Leaping
Ancillary Pool: Electricity Mastery
Hero Profile:
Level 1: Caltrops -- Slow(A)
Level 1: Pistols -- Acc(A), Dmg(15), Dmg(33), Dmg(33), EndRdx(33), RechRdx(34)
Level 2: Dual Wield -- Acc(A), Dmg(3), Dmg(3), Dmg(15), EndRdx(34), RechRdx(34)
Level 4: Empty Clips -- Acc(A), Dmg(5), Dmg(5), Dmg(11), EndRdx(36), RechRdx(37)
Level 6: Acid Mortar -- Acc(A), RechRdx(7), RechRdx(7), RechRdx(11), EndRdx(48)
Level 8: Force Field Generator -- DefBuff(A), DefBuff(9), DefBuff(9), EndRdx(48)
Level 10: Swap Ammo
Level 12: Hasten -- RechRdx(A), RechRdx(13), RechRdx(13)
Level 14: Super Speed -- Run(A)
Level 16: Bullet Rain -- Acc(A), Dmg(17), Dmg(17), Dmg(31), EndRdx(37), RechRdx(37)
Level 18: Seeker Drones -- ToHitDeb(A), ToHitDeb(19), ToHitDeb(19), RechRdx(27), RechRdx(27), RechRdx(31)
Level 20: Poison Trap -- RechRdx(A), RechRdx(21), RechRdx(21), EndRdx(48)
Level 22: Triage Beacon -- Heal(A), Heal(23), Heal(23), RechRdx(40), RechRdx(40), EndRdx(42)
Level 24: Maneuvers -- DefBuff(A), DefBuff(25), DefBuff(25), EndRdx(40)
Level 26: Tactics -- EndRdx(A)
Level 28: Executioner's Shot -- Acc(A), Dmg(29), Dmg(29), Dmg(31), EndRdx(45), RechRdx(46)
Level 30: Vengeance -- DefBuff(A)
Level 32: Web Grenade -- Acc(A)
Level 35: Trip Mine -- Dmg(A), Dmg(36), Dmg(36), EndRdx(43), RechRdx(43), RechRdx(43)
Level 38: Hail of Bullets -- Acc(A), Dmg(39), Dmg(39), Dmg(39), EndRdx(42), RechRdx(42)
Level 41: Electric Fence -- Acc(A)
Level 44: Charged Armor -- ResDam(A), ResDam(45), ResDam(45), EndRdx(50)
Level 47: Power Sink -- EndMod(A), EndMod(50), RechRdx(50)
Level 49: Combat Jumping -- DefBuff(A)
------------
Level 1: Brawl -- Empty(A)
Level 1: Sprint -- Empty(A)
Level 2: Rest -- Empty(A)
Level 1: Vigilance
Level 10: Cryo Ammunition
Level 10: Incendiary Ammunition
Level 10: Chemical Ammunition
Level 4: Ninja Run
Level 2: Swift -- Run(A)
Level 2: Hurdle -- Jump(A)
Level 2: Health -- Heal(A)
Level 2: Stamina -- EndMod(A), EndMod(46), EndMod(46)
Code:
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Thanks a lot, Garent! I'll take a look at that.
I'm making a Traps/DP defender, and I'm interested in how to build it properly. I'd appreciate some advice on power selection and slotting, and perhaps even some sample builds. Thanks in advance!