I played 1st Ed. a bit, have 2nd ed but haven't used it 'hot' yet. It's quite reminiscent of original Traveller, IMO, with the deadly level of combat and where ex-military freelance troubleshooter type characters seem to be the most playable.
Of 2nd Ed - the streamlined skills are nice, and I like the variants that they did to telepathy where you can 'push' your psi-powers to the limit or play within your abilities without fear of hurting yourself. It's a simple yet elegant mechanic that I hope finds its way into other psi/magic systems.
I also quite like how the core classes have been compressed to 10 levels only - if you want to go higher you multi-class or go prestige. In my campaigns, the non-heroic people generally don't go above 10th level anyway, so why does one need the stats for a 20th level commoner (or Worker, in B5)? It cut out most of the dead levels too, so each level gains some worthwhile class ability.
Starship combat is radically overhauled and reads like a sub-game of starship tactics, but it does allow for more B5-ish starship combat where the players can bark out orders like "Afterburners to Full" and "Launch All Fighters" in the fashion of Sheridan. One neat little innovation here is the idea of a "focus" of space combat, be it a captial ship, jump gate, huge alin artifact etc.. Basically it is a relative point that the action centres around, and can change throughout the fight (a group of alien fighters buzz B5, chased by Delta Squadron. The 'Focus' is B5. The aliens break towards the jump gate. The focus becomes the jump gate.
The Influence rules are a nice addition too, although I'm not quite sure why the designers used 2D6 and not d20 as the dice roll. I guess they wanted the results to centre on the average values, but it feels a bit of a wrrench to use a different mechanic. It could also bring in actions that would feel strange to players used to a more direct, active approach. Instrad of hopping into a starfury and taking on the raiders yourself, call your government and get them to send a squadron. Of course you could, like Sinclair, do it yourself when you don't have to.
As an SF RPG in general, I think you could use it a as the basis for a semi-hard game set somewhere like the Traveller or Aliens universe - one where the PCs don't have a great deal of Kewl Powrz to fall back on and have to think their way out of trouble. The 2nd Ed is to the 1st Ed what the 2nd season was to the 1st - it feels a bit more solid, streamlined and consolidated.