If your character is appropriate and reasonably well-designed, it should not suck.
Ahh, here's the rub.
if your character is appropriate: Yes, I agree. The game should allow the creation of a mounted combat specialist, even though that specialty might suck in certain styles of campaign. That's just a necessary part of building a game that accommodates different styles.
and reasonably well-designed: Whoa, there. This is the part I don't agree with. I really dislike the idea that characters need to be "designed." I want chargen to work like this:
The Rules: What sort of a character do you want to play?
The Player: I want to be a holy paladin who charges into battle on her trusty steed and skewers monsters on a lance.
The Rules: Okay then. Paladin class plus Mounted Warrior theme plus a lance. Here you go.
As opposed to:
The Rules: What sort of a character do you want to play?
The Player: I want to be a holy paladin who charges into battle on her trusty steed and skewers monsters on a lance.
The Rules: Here are 3 classes, 8 skills, and 15 feats that might be appropriate to such a character. Out of those, there are four or five combinations that will be effective. The rest range from mediocre to pathetic.
The Player: Uh... which is which?
The Rules: Figure it out for yourself.
I got more than enough of the latter in 3E and 4E. I want as little of it as possible in D&DN.