So, I've started my voyage into Babylon 5 with The Gathering and as promised I'm sharing my thoughts as I go. After 16 years, and on forum such as this, it's possibly redundant but be aware that my post contains spoilers...
The Good
The set up going forward looks great, you have four races meeting at Babylon 5. Two were at war ten years earlier, one was a slave race to the other and there is one that we know almost nothing about. Plenty of room for tension.
Some of the characters are very well drawn, in particular I thought that the Londo the Centauri ambassador was fabulous, with huge flaws but immensely likable at the same time. In fact all of the "alien"ambassadors were good...it's the humans you had to worry about.
There are lots of plot hooks for the future that you genuinely want to find out more about (the Grey council, why the Minbari surrendered, the "hole in your mind" etc)
There also appear to be some clever character hooks for the future (Garibaldi and Laurel's loyalty to Sinclair, Londo's gambling problem, Delenn's short temper).
The not-so-good
For me some of the acting was SHOCKINGly bad! I think Lieutenant Laurel and Del Varner deserve a special mention for being wooden and cheesy respectively. As does the moment when Commander Sinclair finds the Vorlon ambassador, Kosh collapsed and reacts with a slightly bored "Dammit" (although I grant that this may just be the quality of the audio on my DVD).
There seems to be quite a lot of padding in there. For example the fight scene at the end that goes on much longer than it needs to. And the pointless scene where Garibaldi tells Sinclair that he reckons that Del Varner has something to do with the attack on the Vorlon ambassador could easily have been cut and absorbed elsewhere. There are other moments as well and I reckon this could have been cut by 10-15 minutes without losing too much and actually ending up with a much better show (although as Sindatur pointed out to me on a different thread "JMS didn't do random things often....most things are indeed set up and introduced before they play out" so there may well be things that happened that will become more relevant later).
Summary
If I'm honest if I'd come across this on a wet Sunday afternoon in 2009 then I probably would have found it mildly diverting but I wouldn't have thought it was great, or worried about spending too much effort trying to find out more.
But, I think that you have to put it in context of it being first broadcast in 1993, I think then the CGI would have been cutting edge and a huge surprise (even today, the moment that the Volon Fleet appears is great). More pertinently it was released into a world that had only just escaped from Dallas, Dynasty (although I believe that Knots Landing was still running). When the behemoths of the schedules looked like that...
Overall, knowing what I know, the interest in wanting to find out more outweighs any negative thoughts, so further up and further in. Anyway these are just my thoughts. Would be interested to see if any of that chimes with others. In any event, bring on Episode 1...