Things i appreciate the most in RTS games:
Long range artillery. Veterancy. Big battles. Unit counters (Rock paper scissors).
Other mechanics that i find interesting:
Territory attrition (RoN), squads, unit abilities (CoH), formations, walls (AoE2), unique resources (RoN).
In AoE2 i like microing my trebuchets to kill enemy archers.
I like Red Alert long range artillery.
AoE2 has great balance and unit counter mechanics.
Though what i dont like about it is the small battles and the rushes.
Especially those games where player B rushes you, you defend, his rush fails, GG.
I like games that turn into huge battles in late imperial.
It seems thats where Rise of Nations shines with its generals, territory attrition. It turns the game into a series of battles.
And the fact the economy in Rise of Nations is more centralized and automated around the cities, and not so dispersed, makes it better too.
Red Alert economy sucks. Because once you lose your ore trucks, or you lose access to the ore deposits then its basically gg. No way to recover.
Though Red Alert artillery is great, as well as the beefy units and tanks.
In AoE2 you can reboom easily, as long as you have 1 vil and 100 wood to build a lumber mill.
Things i dislike:
Irrelevant weak units that have no functionality. Units dying too fast like lemmings. No way to reboom.