Let's see...what are the two giants in the RTS world? Starcraft and the C&C series, I guess. C&C is as simple as it gets, it's still built around the tank rush. However, Starcraft, which has three
completely different races, rules the roost. Sure, it doesn't have that many units, but think back to the days where the Zergling rush ruled supreme. So many people whining on the forums day in, day out. Still, it's the most played RTS in the world. However, its down to many reasons that it's heavily played:
* It'll run on everything. Fuck, the DS has the power to run it if it was ported. Thus, poor Asians with crappy computers can still run it fine. Blizzard always makes this design choice and it has always paid off.
* It's really, really well balanced. This is actually important because it's what keeps people playing. If the game wasn't balanced, it would decend into cheese-fests online and even new-to-moderate players would get annoyed because tactic-of-the-month spreads
really quickly.
* It has an engaging single player mode to teach you the units and the various races.
It's not one factor that makes a game popular, it's many. In regards to Spring, BA largely wins due to momentum, IMO. It's not a bad game, all things considered, but it basically took over from AA, which itself is an evolution of the uberhack from TA (IIRC), giving it its starting audience. Other mods can build a niche and CA proves that. A couple of months ago, it could be tricky to get a game, but these days, I can get a full game with a lot of specs every evening. There's even enough players to fill up another autohost, if people would take a hint and stop speccing in the hope that somebody drops
Still, to make this wall of text even longer, there are truths in what people say about CA. Making every unit viable in a wide range of circumstances makes it slightly less newbie friendly. Making every tech level available from the start gives you a very wide range of options, but that can hurt newer players, especially when they're used to T1->T2->T3 in nearly every other game. Still, the wide variety of options and starting stances make it a very fun game to watch, especially with the greater emphasis on skirmishing rather than trench warfare, which is what BADSD normally turns into.