A lot of things I learned from others (or the hard way) have been mentioned already. I do have a few items to add. My primary right now is a scrapper (Broadsword/Enhanced Senses), so bear in mind any bias that may cause.
The more people there are on a team, the more monsters in a door mission. Sounds obvious, but if you solo a lot it's a really big change to be part of a large team. More monsters means potentially a LOT more damage flying around. Consequently, if you want to do task force missions, go on some missions in teams first for the practice. I hadn't teamed recently as my scrapper before Positron TF (like in about 7-8 levels) and consequently died - a lot. I wasn't prepared for the radical shift in damage going from 3-5 to 20-30 bad guys at a time.
Get some abilities that can be used on/for other players. I was the sole melee person in my Positron TF, and we had four blasters. I felt about as useful as sneakers on a cat. I've been picking up Medical abilities since then, and plan to get Leadership abilities later down the road so I can have some more flexibility.
Get familiar with the locations of stores and the best route to them - that way you can visit them quickly without delaying your teammates on multi-mission groupings. Also, youll save yourself time when soloing.
When binding to keys on the number keypad, type Numpad and the number without any spaces between.
If you can touch type, make sure youre in chat mode before typing chat messages. Theres nothing quite like dropping most of your windows accidentally and taking off running towards a mob also. (R to Run, C to open/drop Chat, N for the Navigation Window, T for Target information these are the ones I have had the most trouble with.)
You can give (as opposed to trade) something to a player by clicking and dragging it onto the person.
A variation on the stair-climbing methods mentioned: On the stairs from one level of a zone to another, you have retaining walls which keep you from falling off the staircase. Standing on the steps at a 45-degree angle means you can just run backwards and forwards without the strafe that you have to use on fire escapes. (You hit the wall at an angle and slide along it until you reach the landing, then the same in reverse.)
With Issue #2, there is a vastly improved option for finding team members. If you start receiving invitations from complete strangers out of the blue, its because they have deselected the blue button labeled LFG. (They are then no longer looking at a list of just the people looking for a team.)
That's it, hope it proves useful.
