What will I miss out on if I play Diablo 3 without Reaper of Souls?

I own the base game of Diablo 3. I didn't really like the "post-Inferno" content of the game, so I stopped playing shortly after I curb-stomped the final boss for the n-th time.

I understand now that a lot of things have changed in the base game. There's also an expansion. I don't plan to purchase the expansion, but some of these changes sound like they could make the game more fun for me.

I can guess that probably the new character and act are expansion-only content. I know I'm probably "doing it wrong", but if I go back and play now, what else will I be missing out on if I don't have the expansion?

Here are things the expansion added that you won't be able to do if you don't purchase it:

• The Crusader class. You will be limited to the same 5 classes that originally launched.
• Act 5. The story will end with Diablo.
• Level 70. You will not be able to get past level 60. This means you will also not be able to get artisans higher than level 10.
• Adventure Mode. You will be limited to the story mode. You will not be able to do bounties or Nephalem Rifts.
• Blood Shards. You cannot collect blood shards or gamble.
• New Gems: Imperial gems and higher can be dropped / combined only with the expansion. This also implies you can and will have to increase your Jeweler and Blacksmith artisan level to 12.
• New Legendary items: Some new legendary items will only drop on level 70. And some even only on Torment difficulty.

Note that you will see references to all of these things in the game, but you will just not be able to select them.

To put it another way, if you come back to D3 without the expansion, these are the things that you will get:

• The new loot system, which may or may not benefit you depending on how well-geared your existing characters are.
• The new Paragon level system.
• Modifications to many skills & runes, some of them substantial - but no brand new skills (they are all learnable at lvl 61).
• A few new events, including "cursed chests".
• A number of revised boss fights.

I don't know whether you were primarily a solo or multiplayer player, but playing multiplayer might be difficult as your companions might want to do content that you did not have access to.

Of course, since you already own the base game, the only thing that trying it out costs you is some time and disk space :-)