what's the diff between bad company and other bf's
Nothing on the surface level. Everything on the gameplay-fun level.
Bad Company 1 focused on Rush which turned a bunch of fans off, but then introduced Conquest (with the "cap all flags? Game is over in a minute if the enemy doesn't cap one flag, no uncaps/spawn starting areas" gameplay design which I prefer Conquest to be) later.
Bad Company 2 focused on Rush primarily while Conquest suffered for it. However, the Rush gameplay was ACE A+++ in terms of map design (no maps that weren't unfun to play, compared to later entries where there was a lot of maps people didn't like/didn't want to play on). Did a small "DLC"/expansion that was Vietnam (where the maps were alright, but the gunplay/etc. hindered it but it has it's charm).
The biggest difference between Bad Company and the Battlefield main series is 1) No Jets/planes and 2) No prone. #2 had a bunch of folks STEAMED when it was announced, but after playing BF3/4/1/etc. with prone, there's a certain segment that have come around on the crouch being the only "go low" design.
Also while I'm writing things: There was no scope sway in BC1-2 (or BF1942, BF2) that BF3+ introduced. Plus no Sniper Glint that BF3 introduced to try to stop snipers from camping.
The major problem with BC2 is that the destruction was too much, helicopters (on consoles) could circle-strafe to death, and the map design had a few choke-points (weakening Conquest gameplay) due to the Rush-linear design, but gameplay wise?
A lot of fans consider it the high-point of the series. Older fans consider BF2 that point, but it really depends on what you're looking for. For me, the gameplay-first design (no numerous unlocks. There's like 20 max and you get them all within about 20-40 hours of gameplay, compared to BF3/4 where there is like 100 different unlocks no to mention the accessories are unlocks per gun compared to BC2 where they are ACOG/RED-DOT/4x(sniper)/16x(sniper) for all weapons once that "perk" was unlocked) was ACE and what made the series stand-out.
I could write a bunch but for me it's basically "It'sFuckingFUN,Man".gif