This is a long post, so I'm going to put my comments in bold.
Anyway I've played and enjoyed Generations 2-4. I haven't brought Poke'mon Black/White because all my friends say it is similar to all the previous games and hasn't changed much at all. But you may say to this "Why fix what isn't broken?" I agree Pokémon is very addictive, but nothing really has changed in the main series in fifteen years.
Aside from the much needed aesthetic improvements, the battling is virtually the same. The game as a whole, however, feels new and fresh. All of the Pokémon you'll be using pre-elite 4 will be new Gen 5 Pokémon. You can't access any of the Gen 1-4 Pokémon until you have beaten the elite four (yes, this means no more Zubats and Geodudes in every cave!). This is one of the main reasons why the games feel fresh, you never know what you're going to find! If you look at some of the reviews for the games, most of the authors say it is the most fun to play since Red and Blue
I lost interest because the story has never changed. You are a ten year old trainer who is about to embark on his first adventure in what ever region he is in. Your trainer gets a rival and chooses from one of three "Rare" Poke'mon. After this you collect 8 badges, defeat some evil team, capture some legendaries and defeat the elite 4. This hasn't changed through Generation 1-5.
The story in Black and White is considered to be much deeper, darker, and overall better than than the previous games. I've only played to the fourth badge and I'm still intrigued by the storyline. Sure, the main "skeleton" (getting the 8 badges and beating the elite four and such) of the story has not changed, but this is also true for other great series. Take a look at Zelda: You start with only a sword and complete a set number of dungeons to get to the final boss (usually Ganon) and save Hyrule and its princess from evil. The core story remains the same, but each game manages to feel "different" and that is what Black and White have finally accomplished for this series.
I'm going to suggest some quite radical things in the ways of story here. So please bare with me.
1. Give the hero a supporting cast. I personally would like a kind of fellowship whilst I adventure in a world. I think it really helps to add depth to the world and make the adventure seem less lonely and more personal. On the world screen they could be seen walking around with you (Or not what ever.) They could have other uses as well, like when in Double battles their Poke'mon could be in the battle. There may be a romance sub-plot, or what ever really.
Black and White actually do have a supporting cast: Your two friends Cheren and Bianca. You interact with them in literally every town, battle them regularly, and often team up with them against Team Plasma
2. Change the story formula a bit. I mean Colosseum and Gale of Darkness done a good job mixing up the story (Although I despised Shadow Poke'mon.) and I the endings weren't completely predictable. How about a medieval Pokémon? (Lol this ain't a suggestion, I would like it though.)
The really radical story changes are mostly what spin offs are for. The core story of the Pokémon games should remain relatively the same. Though I think Black and White were good starts in "mixing it up"
3. More sub plots. The amine is a good place to take inspiration from here. In the games there is a very small amount of sub-plots. You simply get on with the main story most of the time. How about a starter (Sub mission.) like when your starting out, to go into somebodies basement and kill a few Ratata's (Sorry it's been a while, I used to know all their names. Now they escape me.)
I really like this suggestion, it would give the games much more depth (even more than they already have to begin with)
4. The only game play suggestion I would offer is to make status changing moves more useful. At the moment it's nearly always more advantageous to use an attack move or a move that inflicts paralysis or poison. Tail whip not that useful.
When you use status changing moves on yourself (such as Calm Mind or Swords Dance), they become incredibly useful in game and are vital for competitive battling
I think Gen 4 (which sucked) might have left a bad taste in your mouth. Give Black and White a shot! Chances are, you won't be disappointed.