I've played T1, T2 and T4... of these T2 is consistently the most enjoyable. In space, the fed advantage in ships is balanced by klink experience and cloak. Even in ground it's more fun... I just see a lot more close matches and crazy comebacks in T2 than any of the others I've played.
Also, in T4, things start to become dominated by BOFF powers, instead of piloting skills. Granted this is just my experience. Do things pick up again in T5? what are y'all experiences?
In T4 I notice a lot of the basic things you learned in the other tiers become streamlined. Agile ships can take advantage of less agile ones, it's easier to jet in and out. I haven't done any premades in T4, but pugs are usually of a better quality. Skill builds are coming to fruition, with skills/power being specifically boosted to some very dangerous levels. Those who didn't plan ahead are going to suffer in this department. RSP is pretty abundant, but if you carry counters to it you'll find a place in focus fires. Only real issue is SNB, but that's largely tier independent.
Ground combat is always new and exciting, you now cycle through more expose skills as fast as possible and hit 2, then z, then 2. As opposed to ground combat in T2, which was cycling through all your expose skills as fast as possible and hitting 2, then z, then 2.
I look forward to T5, becoming frustrated, and leveling a pvp Fed until my subscription runs out.