I say this quite often, but I think it's a crying shame that Gundam isn't more readily available in the UK. It's arguably one of the most famous anime franchises of all time, but as it stands at the moment, there isn't a single Gundam DVD/Blu-Ray currently in print.
It's great what Vertical are doing with the Origin manga releases though, and I'd definitely argue that as a fantastic entry point for the franchise as a whole.
For me, I guess the appeal works on a number of levels:
Cool mecha action/shiny mecha suits - you can drool over these and the countless various models like you would flashy sportscars.
Sci-fi - more the backdrop I suppose, but this is the glue that helps support the show as a whole.
Solid war story - Gundam is, at heart, a war story (more so than it is a science fiction one), and it delivers everything that war brings with it, from the cold, stark reality of the situation as both sides fight it out, to the emotional resonance of this amongst both military and civilian personnel.
Humanity (and what humanity means when it's pushed to its limits) - What does it mean to be a man? Will the future see us transcend what it means to be 'human'? What are the limits any one man/woman can go to, and can they change the destiny of the race as a entirety?
Love - Certain series play this up more than others, but certainly in stuff like SEED where it's pushed more to the fore you get shades of harem romance, and all the tensions and 'best girl' arguments that come with it.