I'm slowly levelling a DP/Cold through the Praetoria stuff right now, currently lvl 18.
You're right about the meh'ness early on, I had to 6 slot the T1 and T2 blasts with recharge TOs just to not make it feel so slow, and then went with 3 rech and 3 dmg DOs. That said Praetoria doesn't give anyone favours (oh god why do the ghouls resist slows?).
It's solo viability is fine once you get suppressive fire, or even slotting snow storm for slows until you can get sleet.
One nice thing about cold is that it is stupidly easy to softcap. Even if you cannot afford to invest in that, just taking arctic fog, weave, maneuvers, scorp shield and the steadfast +3% IO will put you at a very respectable 38% S/L, and 19% against all positions. HoB can temporarily push your positional defence to a solid 30% (which is just about the only synergy the two sets have), and all these toggles are completely sustainable thanks to heat loss.
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Just started this as well myself. So far it's pretty fun, figured cold would work alright while solo and help with some really nice debuffs when I do happen to group up. So far with it though, all I can say is how much I love infridgidate in the beginning, greatly helps keeping big mobs away from me while I pick off the minions.
Limitations? Solo viability? Etc.
I haven't seen much mention of it.
I went with this rather than the more thematic Pistols/Traps.
Have I made a huge mistake?
After a night of playing... eh. I'm only level 6. Not wowing me just yet, but I've played /Cold before and know that it doesn't do anything too impressive until the late 20's or so.
Thanks for eight fun years, Paragon.