Whatever happens to matter to you personally...
Not everyone cares about fast flash duration or fast recycling, for example these things don't matter at all to still life photographers.
The cost and availability of light modifiers is a big one to most people, it's no good buying a flash kit that doesn't have interchangeable modifiers, or if it's impossible to get a good range of them, or if they have silly prices. Fortunately, for the amateur market there are plenty of lights with Bowens S fit or Elinchrom fit - IMO Bowens is a lot better than Elinchrom but both are OK. If buying Bowens fit though, make sure that they really are Bowens fit, there are plenty of lights that claim to be Bowens fit that aren't.
Modelling lamps - very important to some people, not to others. Personally I don't see the point in any flash heads that have dim modelling lamps, because if they aren't bright enough to do the job then they aren't worth having at all. Bright modelling lamps usually mean efficient fan cooling too, although there's one make that doesn't have fans on all of its models, relying on heatsink instead.
Then there's colour consistency and flash energy consistency, unfortunately unless you can believe the seller this is a difficult one, because some people tell lies about their products - and anyway, unless you can test these things with both a colour temperature meter and a flash meter before buying you won't know whether the figures, if published at all, are true. Which makes Kris's comments relevant, some firms can be trusted, some can't. As a sweeping generalisation, the Ebay sellers can't.
Go to the shop and try them out, otherwise you can't really compare them.
Fine in theory but not in practice. Most shops don't sell any flash lighting at all, and the ones that do usually only sell the brands that give them the highest profit. Shows like Focus on Imaging are a real opportunity for buyers, but of course the exhibitors are only selling their own products so you won't be able to make side x side comparisons.