Probably too late to the party, but i am bored anyway, so....
1. My favourite RTS is Rebellion, so i am looking for games like that. The special thing about it is the scale, combined with the fact, you not only deal with classic RTS elements like combat and harvesting, but need to do all that fairly complex research (in terms of RTS), then there is trade, culture, diplomacy etc... simply it makes you feel bit more like an emperor, rather than fleet admiral, what is sort of unique and i absolutely love it. The only other RTS games ever to give me similar feelings were sort of Cossacks and Rise of Nations.
The other important aspect for me are factions. The game need to have interesting and exciting factions to play...otherwise my interest plummets hard... with Sins, such faction are Vasari Loyalists... i consider all their stuff related to mobility awesome. Not to mention they boast visually very attractive units like Vorastra, Orkulus, Kortul, Vulkoras, etc...
On other hand i bought 2 RTS games this year - Grey Goo and Act of Aggression. Both turned kinda meh for me and one of the main reasons are shitty boring factions. i would not mind basic oldskool small scale gameplay, but both games give me just bunch of run-of-the-mill units to play with, some tanks, hovercrafts, airplanes. There is very little variety, no real surprises, as usual no naval gameplay.... i recall Zero Hour or even CnC3 being far superior games in this regard...
I am yet to play Ashes, or see some actual gameplay footage other than those released AMD related videos, but somehow i have a feeling i might get disappointed initially here as well. At least until navies are added, or maybe another faction. Hope i am wrong though.
2. Slow build up until I have overwhelming forces - as most people over here i guess. LOL or maybe until i build up the stuff i want to play with - exciting T3 units, experimentals, titans, etc...
3. More resources, but not way too much. And i need to have overview how much i have. Act of Aggression was specially shit at this during beta before they improved their UI - at times i have no idea what i can buy and if i have money for it, because it was not meant to have one single resource pool for you as player, rather several ones connected to the storage buildings...
4. Titans in Sins are perfect example how i like it. Especially the likes of Vorastra or Coronata, its fun to XP them fo use their awesome abilities.
5. Single player, comp stomps with my friends. Especially those are fun. Obviously not into competitive gaming, too stressful and those kind of games seem to be short, lot of times giving up on lot of exciting stuff, in order to win early.
6. Yes, more units, factions, stuff and tech to research, additional gameplay aspects. The best RTS expansions are ever played were Zero Hour, Rebellion and Dark Crusade for Dawn of War.
7. until i win (or lose). If it becomes boring before that, because of some stalemate, repetitive shit or too long build up phase (i hate building bases in StarCraft, zillions of workers and buildings, CnC was always better in this regard IMO...)
8. Proportionally to the included content.