I really did not like the D6 system. My numeracy is pretty poor and adding up those numbers became excruciating once my character actually got good at something.
The Storyteller system was another that suffered from the bucket-full o'dice problem (especially for anyone who played old-style aberrant), but at least I didn't have to do instant sums all the time.
I really like the simplicity of D20 as a dice mechanic, but I somethimes find it a bit random.
One of the best dice mechanics IMO was the 7th sea roll/keep system. To explain why I think I need to into a bit more depth...
nearly all systems seem to be based on some way of adding together a Stat and a Skill. In D20 system the Stat (Ab Socre) counts for relatively little compared to the skill (this is best IMO as it makes the skill worth spending points on andyou can be good at something without having to have a good 'stat' for it). The problem is that every other Ab score point is effectively useless. The Storyteller system had equal weighting on the Stat (attributes in this system) and Skills (Abilities), which gets around the problem of the ability scores in D20, but means that players nearly always develop the stat ahead of the skill (which I don't like). Out of these two I'd choose D20. The 7th sea system, onthe other hand gave you a number of dice equal to your total pool (stat + skill), but let you 'keep' as many as your stat. Whilst this may seem like it puts the weighting on the stat rather than the skill, in game play it became obvious that this wasin reality the other way round. For the same amount of charcter points in the game you could have 3 points in a Stat and no skill points OR 2 points in the stat and 5 skill points (out of five). Rolling seven dice and keeping the best two is much more likely to give you a better result than simply rolling 3 dice.
Anyway that's my thoughts on dice mechanics. The other side of the argument is the class/level system of D20. There are advantages and disadvantages to both, and neither makes me entirely happy. Personally this used to be the biggest problem I had with old 1st/2nd ed AD&D, but I think D20 comes to a good compomise with its multiclasing system, and D20M/Grim tales this becomes so that problems with the class system (lack of flexibility in achieving your chosen character concept) almost disappears. The compromise on the other side of the equasion is where your character class merely restricts how easily you can spend points in different areas (the CODA system of the new LotR and even, to a lesser extent, the Storyteller system are good examples of this) adding the strong character flavour of a class system and controls to abuse that a points based character creation system is open to. For me, levels are no big problem - I can quite happily take them or leave them, but I can see why some people like them and others don't. I suppose the ideal system would allow you the option, letting the GM choose whether you spend your character points freely as they are gained, or that XP must be spent in lumps as you gain levels, with some or all of your spend being dictated by a class.
So that's my ideal system. It's probably not everbody's. The main comercial advantage of D20, though (over the D6 system, to get back to the point) is the little messy bits that other RPGs would avoid, like feats and prestige classes. Love them or hate them, whenever you produce a D20 product, you can always add in some crunch in terms of new PrCs or feats, thus appealing to a wider market. The $500 basic charge for a licence will be the biggest problem that WEG will suffer from. D20 has done so well (even drawing old cynics like me back to D&D after years of hating it after 2nd ed) because there's so much 3rd part stuff out there. Some of it fantastic, some of it downright s***e, but you can always find soemthing to fit with what you want to do. Unearthed Arcana kind of sums this up for me, lots of options to capture the individual feel you want. UA was a great product because it put everything together in one place (and gave it an official stamp of approval), but nearly everything there already existed in other 3rd party products.
that's a bit of a random waffle on the general subject metter, but hopefully you can get what I'm on about
Cheerio,
Ben